Robo-Fiend is Ultra Lord's arch-nemesis.
It's revealed that Robo-Fiend's allergic to milk, which is how he got defeated in the episode, "Ultra Sheen". Sheen prompty then uses llama milk to spray all over his face, forcing him to yell "No! I'm allergic!". He then runs in agony and explodes into pieces in the distance. This weakness of his was seemingly never brought back up again.
- Despite the fact that Robo-Fiend is a robot, he is lactose intolerant.
- Although Sheen states that Robo-Fiend is lactose intolerant, he yells out that he's allergic to milk. It is unknown which of these is true, or if both are applicable.
- In the episode Send in the Clones, Sheen was playing with an action figure of him and Sheen was making him say to Ultra Lord "You can't catch me!".