Amnesiacs Are Innocent

Children Are Innocent. Amnesiacs have memories as short as children's. Therefore, amnesiacs are innocent -- and may have other childlike, or childish, traits to boot.

If there was a Start of Darkness, the amnesia need only roll back to before then to produce this.

As a consequence, Amnesiac Dissonance is more likely to be a good character fearing having been evil than the other way round.

Because they are innocent and not actively good, it may also lead to Criminal Amnesiac, through being too trusting. They may also wreak all kinds of havoc through innocent unawareness.

Anime and Manga:

 * In Death Note, this happens when Light
 * In Code Geass, C. C. loses most of her memory, becoming the sweet, innocent, illiterate, medieval child she was before recieving her geass.
 * Lucy From Elfen Lied: pre-amnesia, she was basically something between Ax Crazy Yandere and Woobie, Destroyer of Worlds. Post-amnesia, she's The Ingenue and a girl who can barely do the simplest things. When she eventually gets her memories back, things get nastier.
 * Nana, who suffers from "extreme amnesia", from Tripeace is extremely innocent, to the point that he doesn't even know what war is. This is in contrast to
 * The entire backgound story to the manga Zennou No Noa: a large group of people lose their memories in what's called a terrorist attack and are basically made to live in a separate city built under Tokyo. They're even called KIDz, and regardless of age a lot of them do pick up fairly childish traits.
 * Fujiko Mine in the Lupin III Made for TV Movie The Columbus Files turns from her usual Femme Fatale self into a gentle and naive girl who completely trusts Lupin when struck with a bout of amnesia.

Comic Books

 * A Teenage Mutant Ninja Turtles has Shredder lose his memory. Averted in that he still ends up doing evil deeds, just not in costume.
 * Played straight in the story, later invoked by the Daltons in the Lucky Luke story "L'Amnesie des Daltons": on seeing an amnesiac inmate be freed, they decide to pass off as amnesiac themselves in order to leave the prison. Matters are complicated when the local governor decides to send them back to prison for breaking the law by any means necessary: encouraging them to rob banks and trains.
 * Madman is a pretty innocent and childlike person due to being an amnesiatic corpse.

Literature

 * In L. Frank Baum's Land of Oz stories, Ozma takes advantage of this trope by means of the Fountain of Oblivion, a magic fountain that causes those to drink from it to lose their memory. This way, if an invading army attacks Emerald City, all you have to do to render them harmless is to trick them into taking a sip.
 * Niniel is The Children of Hurin is initially this, as when she is found sans memories she needs to be taught basic skills such as speech, and her personality is continually compared to that of a child. She learns quickly though.

Live Action TV

 * The Dolls of Dollhouse, having no memory, are purposely programmed to fit this trope until "imprinted". They're naive, innocent, and devoid of sexual desires... so when some of them start exhibiting traits that break these rules, you know something is wrong.
 * Babylon 5 episode "Passing Through Gethsemane" deals with this topic when
 * Once the vampire was resurrected in Being Human (UK) it was as an amnesiac because the ritual wasn't performed by his Familiar who safeguarded his memories. As a result he's completely free of sin, to the point that crosses and stars of David have no effect on him. Of course, being a vampire he has a Horror Hunger all the same, and being an amnesiac he has no context for what's wrong with him.
 * When in True Blood loses his memory, he forgets his bloody and violent past and becomes innocent. This is responsible for much of the character development in season three and he even states at one point that he doesn't want to go back to who he used to be.
 * In Heroes, super psycho serial killer Sylar gets his mind wiped and, after recovering from a case of the Nathan's, he's a wide eyed pup and a true blank slate. When interrogated by Ernie Hudson, he comes off as completely innocent.
 * Notably averted in the Grand Finale of Smallville:

Video Games

 * Terra of Final Fantasy VI has no memories but her name and is a Technical Pacifist and The Ingenue.
 * Isabella in Advance Wars: Days Of Ruin.
 * becomes one of these at the end of Pokemon Mystery Dungeon: Explorers of Time/Darkness/Sky as a result of being hit by Palkia's attack while escaping through a portal after the heroes and Cresselia foil his evil plans to destroy the Pokemon world.

Web Comics

 * In Sinfest:
 * Tangerine lost her memories through being transformed to succubus. The effect is more child-like than evil.
 * Lil Evil drank from Lethe. It affects his character as well. He even goes by a new name -- Lil Evil is mangled into "Lily." (Then he Used to Be a Sweet Kid.)
 * The titular character of Zap has Laser-Guided Amnesia and is cheerful, naive, and rather childish. His pre-amnesia self, however, was far more scheming, bordering on a Chessmaster.
 * Two Kinds: Pre-amnesia Trace Legacy is a genocidal Fantastic Racist. Post-amnesia Trace Legacy fall in Love At First Sight with a Keidran.

Web Original

 * The Trapped Trilogy: The protagonist in the first game, who turns out to be.