the only villain on staff
hes referring to seeker of darkness, not Ansem the WiseSpoiler ShowDepends on the kids. Him being gentle towards Hayner Pence Olette isn't that out of character. He has been shown to be a caring man before the series started. He took in Ienzo as a child and cared for the apprentices as his children. He became cruel because they betrayed him and he wanted revenge.
Him working with Ienzo to bring back Namine at the end could be considered out of character but he was given a strong desire to make amends when he popped up in blank points so it's just a culmination of that thread.
Spoiler ShowIt could be both someone important to Isa and someone we already met. Isa feels more attached to her so it could be a girlfriend or family or just someone he was closer with than Lea. Because Nomura is willing to have Ventus and Marluxia be from the Union era, having other characters make the jump is possible.
So having her be Skuld who became Isa's girlfriend, adoptive older sister, or a new friend is a possible answer.