i have been wondering if we are allowed to use calvin in normal games that are not the tenth scenario. Is he too broken? Please answer.

He doesn't seem unbalanced - nice fixed starting equipment, though no clues or money and not much random equipment - a superhuman Fight track, and good Speed and Luck, but a poor Will track and not much Sanity means that the right side of the Fight track might not see much use. "Been Through Hell" is theoretically nice, but except against Cthulhu it's possible to go for several games without losing max sanity/stamina, and "Friend Indeed" is interesting but 4/5 means it can't be used too much.

Luck 5/Fight 7 makes a decent gate closer, though.

I'm not using him. He doesn't have a "story so far" or a personal story card, and he's the only one who has a stat that goes to 7. Calvin's state is obviously the result of being in the middle of a huge fight, while all the other investigators have stories that imply that the trouble is only just beginning.