Impressions..
First one looks pretty cool, especially that it'll throw knives.. I like the green and the baggy shirt. I dont know what creature it is though, maybe some backstory.
The second reminds me of a maniac that likes getting blood on his hands, the third im not too fond of. The forth seems interesting, depends on how it's portrayed in the game I suppose.
it looks like they'd blend in into the same game,
oh yeah if i look at the first guy he's like.. I was a dork once but then I drank some green slime from a vial back at the science lab where i spent my valentines day, working.. ended up like dis.. I feel evil, yada yada.. heart becomes cold full of sorrow and feels the need to share his 'pain' with everyone else via hurling glass shards at people! woo.. you did say he throws stuff so I can imagine him throwing glass shards drooping with blood as they get hurled. :3
I've no suggestions on art improvements.. will leave that to the masters !