Suikoden is a really really broad series to pick from. If you're going with friends as a group, which game are they going as? It'd be good to all be from the same game.
Though really, I think you could make an awesome Vikki, like Annie-Mei suggested. The best thing is that she's a character that's in every single game. ^^ Super easy. Her outfit just differs.
Otafest, Animethon, Anime Expo, CCEE
Costumes to work on:
Chie [Persona 4], Linda [Okage],
Ringo [Air Gear], Mamimi [FLCL],
Sakura [Tsubasa], Elizabeth [Persona 4],