I’ll jump on that bandwagon and say Primus (thanks for the name, BTW,) is married and has a number of kids, all of whom are near grown. He’ll be in his mid 40’s, his psionic power keeping him spry. The Genasi are all much younger than he is.
Their parents are probably dead by now, but they were explorers of other planes and went on many adventures to places few have been. Primus was the first child, and was actually born in the Astral Plane during an expedition of a dead God’s corpse.
Growing up, Primus was always a little different, and always seemed to know things he shouldn’t. His smile was unnerving to some, as he seemed to always be wearing it, like he knew the answer to some joke about you that you weren’t privy to.
He married over 20 years ago to an Eldarin maiden, so his four children are all half elves, and each taking after a different season, as seems to be a theme with his family.
They are all in the Feywild, where they usually make their home, though they at times vacation in the Prime Material Plane. But he is very much a Nomad. He travels the planes, usually the Material Plane, searching out new knowledge. But he is often home, bringing gifts and spending time with his children. Soon, the oldest will be old enough to accompany him.
Currently, he is on the Prime Material Plane at a request from Terrus, who asked for his help with their...problem. Always quick to come to his sibling’s aid, he came, and is going to meet Imani, the wizard Terrus directed him to.
He can’t planewalk himself. He relies on the services of a number of wizards he knows. Perhaps Imani is one he is familiar with?
P.S. They are changing the Mystic. It is now officially the Psion, and they are throwing out...well, almost everything as they remake it. If they have reworked it by the time we start and have a working class to play test, I’ll update to the new play test, if you don’t mind. It looks like they’re leaning towards just using spell slots instead of points (but hoping to work well enough with points), and it looks like it’ll be a lot more streamlined, rather than a jack of all trades, master of everything.