Karla from FE7 is a good pick. To get her, you have to force yourself to raise a particular character who is crap to a high enough level to not die in one fight against her (Bartre.) Then you have to wait around until the game is almost over, then bring Bartre into a mission with nothing in it except shops and an arena. Then you take him to the arena and Karla shows up to fight him (for some reason.) Now you have to pray that Karla doesn't critical hit you, which is always a possibility. If your crap axeman DOES manage to avoid death, Karla joins you.
And your reward for all this work? The WORST Swordsmaster in the game. She's not horrible, but the two guys you pick up before her are both easier to get and stronger. She's completely useless, especially considering the amount of work you have to put in to get her.
Also, Namco needs to understand that the only random weapon character who was ever cool was Edgemaster. I just don't CARE about all these other worthless guys. Olcadan, Charade... Why? Really, what's the point? There's already a thing the player can click to select a random character, so what is the point of the random character weapon guys?