Several factors; Character design that appeals to me, the amount of raw damage they can do, the damage they do if there is a meter system (not present in DOA), being viable at in a tournament environment, and they have decent offensive / rush down options. Nyotengu and Mai fit into one or more of those categories for me in the case of DOA.
I main Kasumi because when I saw her in dimensions her sad personality and her appearance and long bright hair and her style got my attention. Generally I choose based on design and style. Whether they're good or not doesn't matter to me, I'll use who i use regardless even if they were absurdly bad or absurdly good. Tier standing and having one particular move or flaw doesn't matter to me.
My main is Ayane, and that decision was made mostly seeing her design and personality on Ninja Gaiden games, where in DOA she is not as cool, but still cool enough, so i just stuck with her.
Kasumi in vanilla because she was always my favorite character. In Ultimate I gravitated to Helena because she was class personified and I saw someone doing a Sarah combo back when she could do 2 4H+Ks and was like GOTTA PICK HER UP. In Last Round Ayane is queen because my gameplay trollish but I don't mean to most of the time and her bitchy taunts
@Nuke-Nin ive actually faced trolly Ayanes a few times lol. one of them kept using the same move and taunt endlessly cause the match was SOOO laggy i could barely do anything about it lol
It's online, the primary way to win is to be trolly regardless of the character. Online Ayanes & Hayates do to tend to taunt more the shittier the connection is though.
The best thing about maining Kasumi and Naotora is that when I use their taunts to apologize for winning that makes alot of people annoyed or angry probably more or so than an intentionally bitchy taunt XD