YES, that's exactly him, thank you so much!

I need help remembering the name of some American governemnt agent who studied a Native American tribe during the 1800s

I was talking with a friend about Native American history, and I
remembered there was a guy in the 1800s, I'm pretty sure an agent of the
Bureau of Indian Affairs, who was sent out to study a Native American
tribe in Oklahoma (definitely somewhere in the Great Plains). He ended
up fully integrating himself with the tribe though, to such an extent
that he became a warchief, and helped lead a raid on another Native
tribe, which seriously pissed off the Bureau of Indian Affairs as I
think they also had an agent studying that tribe? He ended up having to
go back to the East Coast, but briefly came back to try and help them
fight back against the government breaking up their land for use in
private agriculture (in court of course, not literally fight).

If anyone could help me find this guy's name, that'd be greatly appreciated.