Name a character you think was derailed.

As in Character Derailment.

Existing examples:
One of the problems with Final Fight: Streetwise is that Guy went from a crime fighter to a yakuza leader.

Rockbiter went from a mostly serious character with some moments of tragedy in the Neverending Story to a doof called "Rockchewer" who has family scenes in a sitcom knock-off,  and sings an ill version of "Born to Be Wild" in the third film.

Adam Malkovich was a reasonable authority figure in the obscure Metroid Manga Prequel(Say what you would) to the first game and at least the final moments of Metroid Fusion.
In the manga, he even encouraged Samus to leave service and become the bounty hunter she is now.
But in Metroid: Other M, HE'S THESE TROPES:
Designated Hero
Unintentionally Unsympathetic
Jerk Sue
Jerk with a Heart of Jerk
What an idiot
Abusive Parental Figure (construed as an abusive surrogate parent anyway)
Informed Ability
Obstructive Bureaucrat (Well construed as one)
Genre Blindness
Heck it was retconned so that Samus left Adam in defiance instead of encouragement from him, which might make the derailment just bad as it is or worse.
He's also less helpful in Other M than in Fusion.
The derailment as a whole can be construed as one of the game's problems.

Now for something I consider character derailment:
Beans the Cat went from having something resembling a personality to being a feline version of Buddy in shorts directed by Jack King.

When Ajin's artist became the new writer, Kei became a sociopath.

It's definitely character derailment when any character becomes, does, &/or holds one or more of the following tropes:
Angst, What Angst?
Coitus Ensues
Designated Hero
Designated Love Interest
Designated Villain
Distress Ball(This could be a sure sign of badass decay and/or chickification)
Dull Surprise
Eight Deadly Words
Failure Hero(Same problems as the distress ball above)
Faux Action Girl(Again a sure sign of chickification)
Hero Ball
Idiot Ball
Informed Attribute(Same problems as Failure Hero and Distress Ball)
Jerk Sue
Moral Dissonance(A sure sign of a jerk sue, and, worse, a designated hero it can also make characters unsympathetic when not intended)
Positive Discrimination(A sign of badass decay for males and non-minorities)
Stupid Sacrifice(A sure sign of the idiot ball)
Unintentionally Sympathetic(This can be a sign of a designated villain)
Unintentionally Unsympathetic(This can be a sign of a designated hero &/or jerk sue)
Villain Ball
Wangst