Perhaps it is Bradamante but not though, and Bradamante and Nael pulled the old switcheroo on everyone; He thinks of whoever it was he slew in her place and ended up looking like the fake Bradamante, or it isn't Bradamante because Bradamante isn't Bradamante anymore. Bradamante...

Based on what we know of Nael I'd say it has to be related to Bradamante in some way though, either that or we have literally nothing to go off... Perhaps Nael orphaned a child when he slew Bradamante and he was thinking of her? If it really isn't Bradamante then that's the only thing I can think of which isn't just a complete guess. Whomever Bradamante was should have been important enough to him for him to carry around a constant reminder in his weapons name, assuming he named it - could have been named by those around him with him not really caring.

Actually, another possibility is that it was someone from Bahamuts past... Bahamut changed Naels form to something more familiar, rather than Bahamut messing up because Nael was thinking of someone.