An IMC Boss.
Richter is the third boss encounter in Titanfall 2's campaign in the mission "The Beacon". His Titan is a Tone class and he is the only boss using a variant of Nuke Ejection. It's not exactly Nuke Ejection, as his Titan detonates, but he does not eject.
Richter is not "strictly" an IMC pilot, and is instead a pilot for hire.