I main Jak because he was the very first character I got good with and I like air f+Square!
Going to main Kat because I love her playstyle and she's probably one of the best videogame character designs I've ever seen.
I main Parappa because I enjoyed the series, and his playstyle happens to be similar to how I like to play. It's fun to add some variety to the online matches, since the human characters seem to be a bit more popular.
I main Sackboy largely because I love LittleBigPlanet, but also because he has an interesting, unusual moveset. He's not very combo or melee heavy, but instead has a lot of odd traps and gadgets which are fun to use. Also, I like using a character that isn't very popular online.