Mean Former Friend

This occurs whenever a good aligned character (sometimes The Hero, The Mentor, the Supporting Leader or the Big Good) is revealed to have had a history as friend, ally, mentor, or even student with a major antagonist or a villain. Not all mean characters are evil because they frequently considered neutral characters.

Usually the friendship fell through simply because the future antagonist went power-mad and the other character did not want any more to do with it, but more complex and unusual variations exist.

Sometimes it will be known all along, but sometimes it will be the subject of a dramatic reveal that can lead the hero and the audience to reevaluate what they know about both characters.

Examples

 * Professor X and Magneto in most versions of the X-Men comics and media.
 * Superman and Lex Luthor, in some versions of the Superman comics, specifically the live-action TV series Smallville.
 * Harvey Dent is shown as a close friend of both Batman and Commissioner Gordon before turning into Two-Face, particularly in The Long Halloween.
 * Black Mask and Hush were both Bruce Wayne's childhood friends, having turned into supervillains in their adult lives.
 * In the Fantastic Four comics, Reed Richards and Doctor Doom (originally known as Victor von Doom) were college students who could only find intellectual debate and stimulation from each other, until Reed pointed out a mistake in Victor's schematics which was part of his last step in his Start of Darkness.
 * Lots-O' Huggin' Bear was this to Chuckles the Clown. Witnessing Lotso become a monster resulted in Chuckles becoming bitter and cynical.
 * Hector and Ernesto de la Cruz used to be best friends and stage partners that would go on tour until Hector decided to quit show business and go back to his wife and daughter, which led to de la Cruz serving Hector a poisoned farewell drink, stealing Hector's song book and using it to become famous.
 * Obi-wan Kenobi and Darth Vader/Anakin Skywalker  were friends as they were both master and apprentice in the Jedi Order, but when Anakin was seduced by the Dark Side and became Darth Vader, he ended his friendship with Kenobi and considered him his enemy.
 * Peter Prickly and Dr. Phillium Benedict were close friends at 3rd Street Elementary. Starting up in his first year as principal of Third Street Elementary during the spring of 1968, Benedict plotted to abolish recess in order to keep students in their classes to raise test scores and further his career. Predictibly, the plan did not seem to work well with anyone in campus (including Prickly and Muriel Finster), and all of the students felt miserable after Benedict put his no-recess policy into action. When Benedict still refused to step down, Prickly went to report to the Superintendent about the situation. Though the Superintendent assured the parents and teachers that Benedict's policy will never be carried out in the district, Benedict still refused to back down, leading the Superintendent to demote Benedict and promote Prickly as the new principal. Benedict then accused Prickly of betraying him by stealing his job, despite Prickly's insisting he never intended anything of the sort. Benedict turned to his girlfriend Muriel and wanted her to leave with him, but she refused and called off their relationship since she could never be with a man who despises recess. Breaking his friendship with Prickly, Benedict then vows revenge.
 * Mr. Patel was close friends with the Johnson family, but when he figured that his uncle makes a living by making cheetahs race against greyhounds, he decides to do the same to Duma with the help of Nigel and Abdullah.