Alright, my first idea is a clan leader (youkai or otherwise) of a family that's been allies with the Mitsumines for a while, but more because the head was close with Daichi and Mizuho than out of loyalty to the name. There's been no contact since Daichi died, but with the heir coming of age, the estranged clan would be a powerful ally to have, and their support would lend the heir credibility. He or she will just have to prove to the head that he or she is worth mentioning in the same breath as his old friends.

The other idea would be a younger character, about the same age as the heir, also from an ally clan (again, youkai or not). This character would be sent to marry the heir. It'd be a bit more difficult, since I'd essentially be waiting until the heir was finalized and then making my character to play off the chosen heir's personality and so on, but that would be fine.

Either way, I look forward to working with y'all. =)