Lea and Isa were friends in Radiant Garden. They joined at about the same time but their friendship slowly went bad.
Roxas does not fight Isa in BBS. LEA fights VEN. Ven just looks like Roxas.
Roxas and Namine are speacial Nobodies. They are different from the others. And I don't really agree when you say they look EXACTLY like their Somebodies. Cuz they don't. They do look different.