I was watching some anime, and I have noticed that some characters seem to always tack on -su to the end of their sentances.

Is this some dialect thing? or is there some sort of grammatical purpose?

Examples would be futaba on mitsudomoe, and yukimura on kaichou wa maid-sama (Can't believe I just admitted to watching that show.)

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.

While Tzadeck may be right, he's in a Kinki.

Up here in Sapporo, I've heard it quite often.
My impression is that it doesn't necessarily make you look dopey -- although maybe a little bit on the "cool" side. I hang out with college age people though, so it may not be the greatest level of conversation to base a judgment on...

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.

Tzadeck Wrote:In real life it makes you look kind of dopey and like a teenager.

What are you talking about, pal?

The Prinnies from Disgaea put っす after everything they say.

ういーっす！ （おす）
ちゃーっす！ （こんにちは）
おはよっす！ （お早うございます）
あざーっす！ （ありがとうございます）
さーせんした！ （すいませんでした）
ごっつぁんした！ （ごちそうさまでした）

Practice these daily and you'll soon sound like an authentic 体育会系出身.