It's just desu, dropping the de.  Though, I've never actually seen that show you mentioned, so I wouldn't know for sure.

In real life it makes you look kind of dopey and like a teenager.  I have some students who do it.  It makes them look like they're not good at speaking politely, or that they just don't care too much that the situation is a polite one.

Whatever impression it gives, it certainly gets annoying if it's overused. It seems to me like it's used by mainly young guys in semi-formal contexts.

There seems to be っす-characters in quite a few things I've seen. 松田 in Death Note, the guy from 猫ラーメン, and there's usually a character in Final Fantasy who says っす after literally every sentence. In fact, '...ッス' was the last thing ビッグス from FFVII said before he tragically died. The link seems to be that they're all goons.

The Prinnies from Disgaea put っす after everything they say.