[FairyKing]

Even if the "evil" parts were bolted on after he betrayed SOL, he might have still been the Swordsman of Darkness using a different driver and weapon back when he was still good, as a "dark is not evil" type of character. Although if that was the case, someone really should have kept a closer eye on him...

It could also make sense for Kento's father (IDK if he's the same as the current Calibur, the wiki says there's a second user but I forget if that's been confirmed yet) to have been the previous Espada.