Sleeper actually worked with Flash during a recent Carnage plot, but they're basically poor Eddie having a surprise pregnancy even though Venom isn't supposed to be able to spawn after the Life Foundation kids. They don't have a permanent host, although they've worn a few people around.

Basically, Sleeper is what happens if you let Venom actually parent the spawn and have good influences and things like feeling loved when you're a wee ooze. They mostly stay almost cat-like because they're less obviously a symbiote that way, and also able to look after Eddie's (mostly) human kid that Anne hid from him until Dylan came looking for him.