Harry Potter / The Black Wizard / The Wizard in the Shadows / The Lord Moristar Morinhetar (Black Wizard Darkness Slayer), Royal Wizard of Gondor and Arnor
Harry ends up in Middle Earth after flooing from the Headmasters Office on Dumbledore's advice for a little R&R. Ends up trapped there and with the status of something equivalent to a shadowy avenging angel.
Knight In Shining Armour: Genuinely chivalrous, kind and protective of innocents. Unfortunately, he's also slightly mad and in perpetual danger of falling off the edge of sanity and in some cases, the Moral Event Horizon.
Leaning on the Fourth Wall: Absently notes - when Gandalf is tearing Denethor a new one - that 'The Return of the King' would make a good title for a book or film.
Berserk Button: As pointed out below, hurting his sister is a terrible idea.
Big Brother Instinct: He's the little brother, but god help you if you've hurt his big sister. When it turns out that Wormtongue raped his sister, he beats the shit out of him, then tries to drown him. When he's about sixteen. He also requests - and gets, until Eirian unleashes her revenge - executioner duties for Wormtongue.
Dark and Troubled Past: Lets see... lost his mother, then his father, to Rohirric raiders, then his sister, the only person left who cared for him as part of the pact the Dunlending Chiefs made with Saruman to create his army of Uruk-hai. His childhood nicknames were 'son of a bitch' and 'half-breed', and people routinely beat him up, sometimes trying to kill him while he fended for himself. He went looking for his sister by joining Saruman's forces and is implied to have killed his first man somewhere between the ages of 13 and 15, certainly by 15, then nearly died due to poisoning, then found his sister alive but having spent the last few years being raped by Wormtongue. Really, it's a wonder the poor boy's sane.
Dark and Troubled Past: See that of Emrys, but at the age of around 14, became the only parental figure to her 8 year old brother, then was taken to Isengard to be used as a breeder for Saruman's army. She was then singled out by Wormtongue as his 'personal whore' and raped several times a week for four years.
Took a Level in Badass: big time. When first seen, she's an emotional wreck, for obvious reasons. Then, she grants Wormtongue a Fate Worse Than Death. By From Out of the Shadows, she's a confident young woman with magical powers including limited target telepathy.
Beware the Silly Ones: Sure, he seems mildly crazy and harmless, but you have to remember that he is one of the oldest elves alive, and that he has had thousands of years of experience as a statesman and a soldier.
Big Brother Instinct: genuinely loved his little brothers and is utterly furious with FŽanor for what he did to them.