Kreel is a human Imperial stormtrooper sergeant of the 501st Legion, created by Jason Aaron and introduced in Marvel's Star Wars #10, with the name first given in #12. Originally designated Agent 5241, he was working undercover in Grakkus the Hutt's palace on Nar Shaddaa as a gamemaster, training arena fighters — Luke Skywalker among them — before reporting Skywalker's location to the Empire.
The reward was command of Task Force 99, better known as SCAR Squadron: a unit of individually specialised stormtroopers built expressly to beat the Rebel Alliance's best. It is one of the more successful ideas the Marvel run produced, because it takes the franchise's most disposable soldiers and gives them the thing that made the Rebels dangerous. Kreel fought Skywalker repeatedly, severed his own arm with a lightsaber to escape capture aboard the Harbinger, and remained in Imperial service after SCAR's defeat on Crait.
Catalogue entry. This figure has a verified record here rather than a full write-up — the encyclopedia labels its own depth instead of padding it.