The title pretty much explains itself, here is an example: Nio from Akuma no Riddle, the whole thing has been bugging me for awhile, thanks in advance.
Can't watch the video, but if it's っす (as in そうっすね instead of そうですね) it's not a dialect, just a personal "style" of the speaker. There are countless other "styles" a person can use other than っす, e.g. でげす (which was used by men around the end of the Edo Period to early Meiji Period but sometimes used nowadays by manga/anime characters), でちゅ, which is like baby talk in Japanese, ってばよ, which is Naruto's style, and so on...
Edit: Just watched the video and I confirm that my answer above is correct. The female character said:
It's just her speaking style, which has the same meaning as: