

Frequently overlaps with Sealed Evil in a Can and Sealed Badass in a Can. Compare with Crouching Moron, Hidden Badass, Demonic Possession, Jekyll & Hyde and One-Winged Angel. This is sometimes the reason to Beware the Nice Ones. This creates the odd situation of an "Evil" super-powerful monster whose "Good" not-as-powerful personality is dominant for whatever reason. One possible variation on this trope makes the Evil Side the original personality. This is the converse of With Great Power Comes Great Insanity, seeing as in this case, with great insanity comes great power. Alternatively, the evil side may escape his mind/body altogether, becoming a very dangerous Evil Counterpart. It answers the question "why doesn't he just do that all the time?" with "because then the evil would take over." Additionally, if it leads to a FaceHeel Turn, it can serve the same purpose as Good Is Dumb in reverse - it justifies taking one member of the Five-Man Band or whatever and making him a match for his entire team while he has given in to evil.

The narrative purpose of having a super-powered evil side is that it provides a way of saving a hero who is otherwise beaten but without making the hero look disproportionately powerful. Generally speaking, though, he's usually safe unless he starts thinking Evil Feels Good. Thus, the hero faces the very real threat of not returning to normal, each bout of Alternate Identity Amnesia causing him to get more worried about the next. Keep in mind, of course, that once the Id is out of the subconscious it's not likely to want to go back in. To be technically accurate, this is more of a Super-Powered Alter Id - this personality is the physical manifestation of the character's basic drives without having to deal with the inconvenience of higher thought processes and morality - but sometimes this character will have traits of actually being evil or even worse. Interestingly, notwithstanding the name, this character isn't necessarily evil - he just seems like it because a superpowerful version of a normal person who has no compunctions about randomly destroying things registers objectively as "evil" on most people's scale. Think of it as the Enemy Within when it's not the enemy, or as Jekyll and super-Hyde. He's got glowing eyes, some new Facial Markings, a different way of talking, some new threads, and a lot less compunction about ripping someone in half. Unfortunately, it's not exactly the hero anymore. And then the hero somehow suddenly taps into a massive hidden power source within him and unleashes a can of whoopass on his opponent. He's wounded and at the end of his strength, and his barely scratched opponent looks down at him and sneers. The fight's not going so well for our hero.
