In the early part of X-Men the Animated Series Morph is left behind after the group attacks a Sentinel Facility. A friend of both Wolverine and Beast losing Morph greatly hurts the team. This is not the end of Morph though he reappears but this time as villain who terrorizes the team. It is later revealed that he has been brain washed by Mister Sinister. Similar to the comics Sinister has his eyes fixated on Cyclops and Jean Grey.
Sinister shows up with his own super team called the Nasty Boys and attempts to kidnap Jean Grey and Cyclops.
I love this SERIES!
I even love the music from this series. None of the X-Men cartoon series after this have quite captured the awesomeness of X-Men. In this series Cyclops is actually pretty cool, Storm is awesome, and Wolverine is the best at what he does, and its never being nice. The best part of the series though is that it dives without flinching into the comic stories and stays fairly faithful. There are tweaks made but generally these stories stay fairly faithful to the comics. For this reason we get a fairly "comicbook" accurate portrayal of Mister Sinister.
I hope this version is used a template if and when he is brought to the Big Screen.
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Who is Mister Sinister?