The Legionnaires: Nemesis Kid
Programming note: Next week's LSH review is going to be much later than usual, by, like, a week or so. Sorry 'bout that; unavoidable.
Nemesis Kid aka Hart Druiter of Myar, aka Churl*. Created by Jim Shooter and Sheldon Moldoff.
Legion history is full of people who join the Legion under false pretenses, to try to betray them or take them over or whatever. In general it's not worth counting them as official Legionnaires. Nemesis Kid was one such character, but it's not on that basis that I'm including him here; it's because he joined the animated-series Legion in the Karate Kid episode, and remained with the team thereafter. If he appeared in the cartoon again I missed it, but he did show up in an issue or two of LSH31C and remained a member in good standing when that series ended. Probably, if there had been a few more issues of LSH31C, the writers would have gotten around to having him betray the team in some way, but even if that had happened he had already logged enough legitimate superhero time that I'd still give him an entry here.
Nemesis Kid originally had an adaptable superpower: he had whatever superpower he needed to defeat the person he was fighting. This was formidable, but it had a big loophole: his power couldn't adapt to fighting more than one person. That's original-continuity Nemesis Kid, mind you; threeboot Nemesis Kid's powers were never described, and animated-series Nemesis Kid had the power to neutralize anyone's superpowers, which is a lot easier to parse.
Nemesis Kid was a villain. He was a longtime member of the Legion of Super-Villains, he was essentially responsible for the first death of Karate Kid, and he was executed for his crimes. Nobody misses him. For his signature moment, let's pick the one time where he really did seem like a nice guy. It's about five minutes into this clip:
(This is from the episode where Karate Kid and Nemesis Kid join the Legion. Nemesis Kid is taken on as a full-fledged member, because he's worked with the Science Police and he has real superpowers; Karate Kid is a probationary member. As such, he has to do all kinds of menial tasks around Legion HQ, such as laundry, while characters like Cosmic Boy and Nemesis Kid look sternly at him and generally lord it over him. (The racial politics of this have not escaped me. I have no idea what the writers of the show were thinking.) By the end of the episode, Karate Kid has proven himself as a Legionnaire, and Nemesis Kid offers to do his laundry.)
* not really