It is possible. Just go into the editor and open up the categories pane from the panel on the right side (clicking the arrow on the right side of the screen to open that panel). Then, scroll down to the Categories section, mouse over the category that you want to remove, and click the trash can icon on that category. After you've made whatever other edits to the article you want (which could be no edits), click the Publish button. That should remove the category for you.
I know that the Ominous from Sonic Overload (Or "Bad" Ominous) wasn't that redeemable, but what about the "Anti-villain" Ominous (Or the one that I've already wrote a sympathetic personality for)? Do you think he should be redeem or not, and if so, why or why not? What do you think would be a fitting punishment for him if he shouldn't be redeemed? Thanks for the reply if you wanted to or already did, sorry if I bothered or upset anyone, and have a great day.