To those asking about League's main character: there isn't one, really, but there are a good dozen or so Champions that are in the spotlight more often, for one reason or another.
Some, like Ashe, Ryze and Garen, are seen as very "basic" characters in their roles and can be seen as part of the "OG" crew of League. Then there's the ones Riot likes to push, like Ahri, Jinx or Lux. I'd say there's some margin for discussion regarding which rep they would choose.
However, I do think that for the most part, a mage character would take precedence, because in general they could represent some of the base mechanics of League better in a crossover fighter imo.