We know Bartholomew Kuma is part of a “special” race and I think just one look at his name gives away what that race is. It’s likley that Kuma is a Lunarian.
Kuma is themed after bears, that’s what his name literally means, and bears sometimes have moon/space imagery associated with them. Particularly one species, the Asian black bear. It is known in Japanese by another name, as ツキノワグマ tsukinowaguma, literally “ring-of-the-moon bear”.
If that sounds familiar, it’s probably because Pokemon used this exact same pun for the basis behind the designs of Teddiursa and Ursaring (Teddiursa has a cresent moon, Ursaring has a new moon, and the new evo Ursaluna has a full moon).
So we have some ties with the moon theme, but there also some other subtle hints. Kuma carries a Bible which ties into some of the theming behind Lunarians (they look like angels, and now we have the Seraphim, an actual class of angels built from their DNA).
His appearence is the biggest question mark, I think as a child he may have cut off his wings and dyed his hair to escape persecution. Wouldn’t be the first time someone tried to hide their race, we just saw it last arc with Kawamatsu. I think it’s possible the Seraphim Kuma is just how he actually looked.
It’s likley Kuma was a Lunarian king who was discovered, and so the Goverment kidnapped his daughter to force him to turn himself into a test subject, thus creating the Pacifistas and Seraphim out of his DNA.