Four reviews for Joe and none for Carolyn, the most interesting investigator in the game? This will not stand.

The Combat Medic

Carolyn Fern is an especially durable Guardian with a powerful ability and effect. Additionally, her deckbuilding options are extremely broad, enabling great flexibility in her builds. This is both a blessing and a curse (appropriately), as one can feel inundated with decisions and suffer from option paralysis. Additionally, the devs seem to value flexibility very highly when creating characters, resulting in Carolyn having a base stat total of only 11. Her increased durability (she has an extra point in health/sanity) offsets this, but I would say a stat point is more valuable than health/sanity, so it's a bit meh.

What Makes Her Great

Carolyn's deckbuilding options combined with her ability open up a massive array of options that totally warp how a player should value cards. What do I mean by this exactly? Take First Aid, a fairly lackluster card that few Guardians can find room for. In Carolyn's hands, it can not only bring an ally from the brink of insanity but also allow them to afford their expensive assets. Clarity of Mind becomes a similar tool for both saving and paying allies, despite being maybe the worst horror heal in the game overall. Once higher level cards become available the the player, one's options expand into truly ridiculous healing tools like First Aid(3) and Ancient Stone(Minds in Harmony). On the other hand, those two items are basically the only high-level horror heals that your allies will care about.

A sidebar on Ancient Stone: With Shrewd Analysis, one can upgrade two copies into Ancient Stone (Minds in Harmony) for only 3xp. This is well worth one / slot, especially since the stones free up their slots after being upgraded. Identifying it can be tricky without devotion to clue-gathering, but shooting for a total of 4+ is very strong, since it neatly counters Rational Thought.

Basically, Carolyn fundamentally alters the value of many sub-par cards, increasing the amount of viable includes while deckbuilding.

Core Builds

Carolyn has so many options that it can be helpful to focus on a strategy and include some core elements, filling out the rest of the deck as one sees fit. To that end, here are some effective cores:

Cluever

Dr. Milan Christopher
Alyssa Graham
Magnifying Glass
St. Hubert's Key
Deduction
This is a build with very strong static increases to investigation. With everything in play (which ostensibly includes Charisma, a superstar include for Carolyn) the investigator is up to 8 while investigating. This would require a massive amount of resources, but I have done it. It also makes identifying the stones fairly easy.

Support

First Aid
Clarity of Mind
Liquid Courage
Inspiring Presence
With these assets Carolyn is able to bankroll her allies, feeding money to the fighter so they can afford their expensive weapons. With xp, Stand Together is an incredible value both xp and card-wise. Inspiring presence can also be used to heal and recharge your own or your allies' Beat Cops.

Combat

Guard Dog
Beat Cop
Shrivelling
"I've got a plan!"
Dynamite Blast
This build uses guaranteed damage to deal with enemies and Carolyn's tankiness and healing to soak shots. Between her ability, Stick to the Plan and Emergency Cache, she has consistent access to guaranteed burst damage, despite it being fairly resource/xp intensive. In any case, "I've got a plan!" is and has always been a very strong outlet for damage, despite its one-shot nature, and I feel Shriveling would need a lot of support (from St. Hubert's Key/Holy Rosary, Arcane Studies, and/or Peter Sylvestre(2)) to be effective late-campaign.

Other Strong Cards for Carolyn

Peter Sylvestre
The big man on campus can provide you with a constant stream of resources, while providing his static stat boosts and infinite horror soak as usual. Combined with a reliable way to take horror, Peter becomes an enormous economy card rivaling Dr. Milan Christopher. Speaking of which...
Forbidden Knowledge
With Carolyn's ability to heal horror, this is very safe to take. If Peter Sylvestre is in play, you double your resource gain and take no horror, which is quite strong. Cautious players (or those on higher difficulties) might opt for Painkillers instead, since it can provide fast, reliable horror in much the same way while shoring up Carolyn's low health.
Charisma
In every Carolyn deck I've ever built, I've ended up with more than four ally cards. Especially if you decide to go with Foolishness, one or even two copies of Charisma will enable Carolyn to maintain powerful effects on the board, such as Dr. Milan Christopher and Peter Sylvestre, or Alyssa Graham and Dr. Henry Armitage, or Foolishness and Peter Sylvestre. Her allies are incredibly useful.
Ever Vigilant
Similar to Charisma, Ever Vigilant allows Carolyn to setup for the whole game in a single action. I prefer assets over events for horror heal, so every deck I build with Carolyn has more than 20 assets. In this case, Ever Vigilant is incredibly strong and will likely be useful even late in the game, especially if you've found yourself without actions to play cards and great assets building up in your hand. This can also go under Stick to the Plan, although Carolyn doesn't have quite as many viable options for that card, so it might be overkill. I think taking aggressive mulligans for Ever Vigilant is probably the better strat, as it saves you 6xp.

The most effective strategy is probably to combine two of these cores into a deck that can take useful actions at any point in the game, for example by combining the Cluever with the Support suite. Or one can be more adventurous and combine everything they think is cool into some kind of monster. The options are quite vast.

Signature Cards and Weaknesses

Hypnotic Therapy is a very good card. Being able to heal one horror from any investigator every turn for the rest of the game is ridiculous, and it draws that investigator a card, and you can give them a dollar with your ability. If this is the only horror heal you get in play, things start to stabilize instantly. If you get it into play later, it's a steroid for your other horror heals, so that's great too. I wish it didn't exhaust for both effects, but it would definitely be broken if that were the case. Rational Thought, her corresponding weakness, isn't nearly as bad as To Fight the Black Wind, especially with Hypnotic Therapy or Ancient Stone (Minds in Harmony) with four secrets on it in play. It is still very annoying, since you don't get the benefit of your ability while it's out, so it can hurt your income. In my experience, it hasn't affected me at all, but I have yet to draw it when we're all about to go insane and I'm low on healing options. In any case, Alyssa Graham is useful to avoid such occasions.

Foolishness, her replacement signature, is very good, but also very annoying. Aside from needing a bunch of support to come online (which, admittedly, pays you back a bit), he also takes the ally slot, which is so incredibly valuable. I think he becomes more attractive in a four-person party where other investigators need Dr. Milan Christopher or Peter Sylvestre more than you do. To Fight the Black Wind, the corresponding weakness, is extremely terrible and can result in the agenda flipping a turn early if drawn in the upkeep phase. Ancient Stone (Minds in Harmony) is also very good against it, as long as you draw it with some actions to spare. There's certainly an argument to be made for taking all of her signature cards, since they do have some synergy with each other, but it is resource intensive.

Conclusion

Carolyn is one of my favorite investigators, probably for the same reasons that many seem uninterested in her: she's super weird. Her versatility is incredible, her ability is interesting and strong, and has no phase/turn limit, and her stats are strong enough that she's a competent investigator before applying any bonuses. Still, she has her downfalls: I've frequently found myself in scenarios with no horror to heal, which makes me value Forbidden Knowledge a little more highly. Her effectiveness is also highly campaign-dependent, I feel; she has plenty of horror to heal in NotZ and Carcosa, but less to do in Dunwich. In Forgotten Age, I think she's downright bad because the campaign throws far more damage at one than horror and her is garbage. With its focus on "the ethereal," I'm going to assume the Circle is also heavy on horror, so she might function quite well there (investigators are generally pretty powerful in their release campaigns).

Anyway, I hope you've enjoyed this review of Carolyn Fern. Thanks for reading.

Edits:
Carolyn can't take Monster Slayer; I'm an idiot. Good catch @Death by Chocolate.
Added Dynamite Blast in the Combat build; she's one of the few s who can afford it, and I forget how good it is; good call @Swekyde.

SGPrometheus already wrote a definitive guide to Carolyn, but with the Innsmouth conspiracy, she's playable in a fourth archetype, blendable with the other ones. Holy Roller.

It's entirely due to the Book of Psalms. A card that’s decent for others is a godsend* for Dr. Fern, paying for itself +1, healing potentially eight horror from your teammates with hypnotic therapy in play (more with knowledge is power).

Add Rite of Sanctification, and spiritual therapy is very possible. Your super expensive assets are now much cheaper, adding 10 bucks to the party with an action and a card. You can pair this with more support or clue hunting, because a better bag makes you more capable of anything. Get out your appointment book, there’s a new way to play Carolyn.

*Ha ha?

Sure, Carolyn Fern likes to heal horror, and is incentivized for doing so, helping the whole team out with a pile of resources. But what about healing damage, which is also a problem in some campaigns? There are some good Guardian Level 0-3 cards that do just that, like Second Wind and Emergency Aid. But how about we take a look at which cards can heal both damage and horror, to save ourselves on deck slots?

• First Aid (0): heals damage or horror, one at a time, only from an investigator.
• First Aid (3): A big glow-up that heals one of each, from either an investigator or an Ally at your location. You really want this one for two reasons -- (1) many enemies do one damage and one horror as their attack, and if a hunter that has that attack is on its way, you can save yourself from dying for one more turn. (2) If your Ally or a teammate's can take damage and horror without being defeated, you can use this to keep them from dying forever, maybe even to reuse their abilities every round (hello, Agency Backup!). Since both horror and damage is being healed, the target's controller gets a resource on every use. Oh, and it uses supplies, so Emergency Cache (3) is on the menu.
• Medical Student: Like a one-time First Aid (3), and then will take one more hit.
• Hallowed Mirror: the Soothing Melodies are like a more flexible First Aid (3) that draws you a card, but you only get three scattered through your deck. The upgrade adds an extra point or reshuffles it back in.
• Call for Backup: one use event, can split the damage and horror healing across any two cards, anywhere, and you'll need Peter Sylvestre and a asset in play, but you should do that anyways. Strong if you can get all of the other effects, too.
• Earthly Serenity (1)/Earthly Serenity (4): Another spell for Carolyn to upgrade! Only targets investigators, but very versatile. You do need to make a test, though. This is less good for Carolyn since you'd like to heal one horror point at a time to give out the maximum number of resources; BUT you can succeed by 3 and heal two damage and one horror. And sometimes burst healing is necessary.
• Thermos: It's level 0, but gets more interesting if everyone has two or more trauma. Carolyn can probably afford the resource cost. By the time this is a viable card, you should be able to afford First Aid (3) or something else.
• Archive of Conduits: This is like an unlimited First Aid (0) with the huge benefit of the targeted player getting to draw a card (something like Liquid Courage (1)), AND the targeted investigator can be across the map. Sometimes you'll get to burst heal. Doesn't work on Ally assets, though, and Carolyn can't give out resources if the action is used to just heal damage.
• Inspiring Presence: Great if your team has Ally assets to heal! You'll be making and tests anyways.
• Lonnie Ritter: heals damage from the team in a roundabout way. Only you will get the resource each time Lonnie heals, essentially refunding the cost of healing.

A few cards only heal from you, so they don't really help the team but are worth a mention:

• Precious Memento/Precious Memento: Only heals from itself, which is fine, but doesn't really help out the team with resources. Let's put Spiritual Resolve in this category, too, since none of these trigger Carolyn's resource ability either (Also true of healing horror off of Holy Rosary and the like, but that is much less likely to happen).
• Deny Existence (5): You want this because all of the other effects are strong, not because you're planning on healing damage exclusively. Again, only heals you.
• Spirit of Humanity: Well, it doesn't take an action to heal, so I guess that is good if you don't mind the tokens. But this card is probably useful for adding the tokens in the first ability and counteracting the damage with some kind of Lonnie or Peter shenanigans.
• Meditative Trance: Ehhh, maybe if Carolyn has Shrivelling and something else?

So, if Carolyn is going to be a full-on medic, she has a lot of interesting options. With Edge of the Earth's Down the Rabbit Hole, the very best options are available at a discount: First Aid (3), Hallowed Mirror (3), Archive of Conduits (4), and Earthly Serenity (4) all have level 0 or 1 versions.

I recommend choosing one of two routes to maximizing your healing potential: supplies or charges. Depending on what else you're trying to go for, either can give you a rainbow of colors for the Synergy cards Carolyn can take advantage of. If you're going the supplies route, focus on First Aid, and maybe drop those supplies on a Fingerprint Kit or some Liquid Courage while you're at it, or possibly Painkillers to heal yourself, or load up Venturer to help a friend shoot something. You probably want this route if you're going to stay focused on testless healing, damage, and other support, and help out with clue-getting with Carolyn's high . Archive of Conduits will slot in nicely here too.

Otherwise, you may want to boost your and lean into and access to refill your Earthly Serenity with charges with Enraptured or Winds of Power. 9 charges is a lot, so maybe you don't run out, but there are other cards you might want these for, like Close the Circle or Shrivelling. If you're taking other cards with secrets, like Forbidden Knowledge or Ancient Stone, Enraptured can help here too, but it is slow. Maybe Eldritch Sophist can help move charges or secrets around for you. Finally, you can also search for your Soothing Melodies with Arcane Initiate, along with any other interesting spells like Blood Eclipse, Brand of Cthugha, or Deny Existence.

It's more a question / comment than a review, but did anyone noticed that arkhamdb treat the official starter deck as invalid? I guess it is because it doesn't take into account that the 15 Seeker/Mystic limit should only be applied to cards that doesn't "heal horror". That might be a problem when trying to save / publish self made deck too.

I have a question about the card slots during upgrading. Carolyn Fern is permitted 15 Seeker/Mystic cards that are non-healing. At the start of a campaign, let's say you choose 15 cards.

After Scenario 1, you spend 2 XP to buy two copies of Ancient Stone. The unidentified version does not say "heal horror", so am I correct that we need to remove 2 of our 15 non-healing Seeker/Mystics cards to add the stones to our deck?

If yes, then once we identify the Ancient Stone, and we then upgrade them to the Ancient Stone, since these have "heal horror" text, then we are now have only 13 of our 15 Seeker/Mystic card slots filled. We have 2 slots available again, yes?

If that's the case, what 1xp Seeker/Mystic card do you recommend to fill that space? Since 0xp cards still cost 1xp when you add them to a deck during a campaign, it feels bad to spend xp on a level 0 card. I'm curious to hear what the community has to say.

With the new ruling on "For Each" effects, where each instance should be resolved independently when you have a choice between each instance of a given effect, Carolyn suddenly has a few cards that can give her or her friends multiple resources in one action. Some of these are:

Meditative Trance: You can break even on this event at 2 slots filled, or go higher if you have extra arcane slots for some reason.

Earthly Serenity has the most obvious direct application, especially the upgraded version. Just play it and get a ton of money.

Logical Reasoning can go revenue positive if you have a total of at least 5 horror amongst investigators at your location - not unlikely in some campaigns.

Liquid Courage, sadly, does still have the "additional" modifier on it, so it should still only give carolyn one resource - unless I'm wrong!