Who do you think was the bigger traitor in Kirby history, Marx or Magolor? (WARNING! SPOILERS for both Kirby: Super Star Ultra and KIrby's Return to Dream Land ahead! DON'T READ IF YOU DON'T WANNA GET SPOILED!)
It's hard to say. Both of them make you do a bunch of tasks for their own benefit and both were really cool bosses with really cool music. I don't think one is less of a traitor than the other!
Honestly I think Magalor is the bigger traitor only because you repaired his ship and after being so nice to him he backstabs you near the end.
Obviously Marx. https://www.youtube.com/watch?v=phJ3Gn8xI2Y I might remake this theme if I have time.
We all know Dedede is the biggest traitor. Not to Kirby, but to villains everywhere. The guy is so funny, it's hard to take him seriously, and he helps Kirby far more often then he hurts him (willingly).
I'd probably go with Magolor! I've never played Return to Dreamland, but in Superstar you only really see Marx once in the whole story before he shows up again at the end of the game, making it not as much of a surprise that he was just using you the whole time. He never does or says anything that explicitly plants himself on Kirby's side, as far as I remember, but just tells him about the conflict between the sun and the moon and sends him off. In RtD it seems like you get to know and develop an actual bond with Magolor, making it more of a shocker when he turns on you.
It's pretty debatable. On one hand, we have Marx, who isn't seen again until the end of Milky Way Wishes, and Mogalor who is seen all the time, but pulls a Heel-Face-Turn later. Marx isn't as shocking when you think about how little you see him compared to Mogalor, but Mogalor didn't exactly stay evil. Going by shock, I'd say Mogalor (ironic since I suspected it long before I even played the game), but otherwise I gotta say Marx.