1. Play as the characters you think are coolest. Get a feel for them and see if any have a play style that works for you. If they do, boom you have a main
2. If they don't you just continue trying all the characters until you find the ones that suit you
So all in all. Its looks and play style. I also personally tend to go for characters that are unpopular. If there's a character people generally view as weaker, I will go out of my way to master them and main them
I'll generally main 3 characters at once. One is always the coolest. Two is always the weird one. The third is another of either of the above
I have a group of six or so people who I play most of my games with. In games with rosters will always make sure we have different mains, so that plays a factor in who I pick. Typically I go for the lamest characters, in smash I was King Dedede and Jigglypuff, because I know my friends won't pick them. If my preferences change they change, but that's how I usually pick.
For me its the first 3 tied together, I always go for the heavy hitters like Sweet Tooth and Kratos. Not only that but their stories of how they came to be are epic and bloody LOL. As far as looks go it depends on their alternate costumes, for example I prefer the normal look of Sweet Tooth over his Starhawk appearance.
Well I normally just choose whatever character is the most fun to play, in SSB I always either played Captain Falcon or Ganon even though they both suck. But it depends if I felt like going beast mode and winning i'd switch to Ike or Falco. It really does depend