Calvin is my favorite gator in the game. That is not to say that he is easy to use. He is not. In fact, his restrictions make games tense, and difficult, and oftentimes disheartening. But this is Arkham. We live for that.
However, when Calvin works, when you put forth the effort, when you grit your teeth, take it on the chin, and get just a little bit Lucky!, you can succeed beyond your wildest dreams.
Plus, Calvin's restrictions force you to build a good deck. Since you need to be toeing the line at all times, you need, need, need to put cards in that help you survive. Peter Sylvestre, Cherished Keepsake, and Leather Coat are all important to staying alive long enough to get you to a point where your stats outweigh your damage.
With the recent release of Nathaniel Cho's preconstructed Investigator deck, Calvin got a ton of new, fun toys. "Get over here!", Glory, One-Two Punch, all slot into a Calvin deck nicely. Plus, you can Versatile in those Boxing Gloves if you're feeling feisty (I do not think this is good, but I do think it's fun.)
Calvin can do almost anything, so long as that anything is his best imitation of a tanky Guardian. You're not likely to be the primary clue-getter, but you are likely to protect your other players with Ward of Protection, "Let me handle this!", Heroic Rescue, and A Test of Will. Run headlong into danger, get beaten up a little bit, and enact your revenge.
Calvin also is the single best investigator at using Rise to the Occasion, as his base stats are zeroes, no matter how much damage you have on your person. This only changes if you become a Werewolf, but by then you're probably going to win as is, so nothing to worry about.
Get a good weapon, get your boyfriend Peter Sylvestre, your girlfriend Jessica Hyde, and your neat playing card Five of Pentacles, and you will be a force of nature. Happy hunting!