Percy Weasley.
Rigid, power-oriented, and driven by interests.
As a senior licker at the Ministry of Magic, he once wrote to his brother Ron, who was appointed Gryffindor Prefect, urging him not to keep in contact with Harry Potter, and asking him to have a good relationship with Professor Umbridge and help her more. After personally assisting in the capture of Dumbledore's Army, he gained the favor of the Ministry of Magic and rose through the ranks.
So, what made him change his mind and help the Order of the Phoenix defeat the puppets of the Ministry of Magic less than two years later?
What makes him willing to give up his obsession with power and glory and turn to the side of justice?
It's him.
The man who treated Percy with insults for three hundred and sixty-five days.
Purpose: Hee hee, Percy is such a weakling that needs to be dealt with.