Final Fantasy 2 was Squaresoft's first try at a game with main characters. Though few in number, each character has their own interesting story, and each have different abilities that make them assets to your party.
Firion is the first lead character in a Final Fantasy game with a name. He is much like the warrior from Final Fantasy 1, and starts out the story for you. He is Leon and Maria's foster brother. After the town is raided and his foster parents are killed, he and the other three join together.
Maria is the first female character in a Final Fantasy game with an actual name. She's like a female warrior from the original Final Fantasy. She is Leon's sister and Firion's foster sister. She joins the other three when the town is raided.
Gus is the kind of guy you just don't mess with. He's like a big buffed up version of the monk (or black belt for you fans of the original FF1) and is a great part of your group. He is a friend of the group and travels with them after the town is raided.
Leon is the really suave brother of Maria and Firion. Despite the fact that the less cool Leon in FF8 is named after him, he's a great character. He too joins Maria, Firion, and Gus after the town is raided.