Here's one I cooked up a year or so ago for a wizard in a D&D group. (I've also been assinging the name and backstory to any "mage" character I can play in a video game RPG.)
Now a decrepit old wizard, Elderast spent his entire life learning from, and then teaching at, a college of magic. He had an aptitude for understanding the theory of magic but had great difficulty putting it into practice himself. Throughout his time both learning and teaching at the college he was ridiculed and publically mocked by his peers, as well as greatly humiliated on multiple occasions. Any time Elderast was not being mistreated he was simply ignored and considered insignificant for his lack of talent.
His father died when he was young and his loving mother raised him by herself in a life bordering on poverty. When his mother became ill, Elderast cared for her until her death, just before his final year as a student of the college. For his entire senior year he kept a lock of her hair with him always and could occasionally be seen weeping with it in the darkened corners of the college halls.
When Elderast was invited to a celebration by his fellow graduates, it seemed his peers were endeavoring to repent of the horrible way they had treated him. However upon arrival at the party, Elderast was once again mocked and humiliated. His classmates held him down while others pulled the precious lock of hair from his hands and threw it to the ground so that all attending could gather in a circle to urinate and defecate on the token of his mother's love.
As a professor at the same college, Elderast was not openly mistreated by his colleagues, but was mistreated just the same, by both faculty and students. No longer because of his lack of talent, but for his socially abrasive and awkward nature, the result of years of emotional scarring. In his career, Elderast was overlooked in every way possible, despite serving longer and working harder at his college than any other staff member. His retirement was ultimately forced upon him by political maneuvering among staff and students, which proved the proverbial straw on the camel's back. Elderast didn't simply break. He snapped.
He withdrew to his shack in the woods and obsessively studied his craft. His passion to learn and master the magical arts was driven by a desire to one day enact revenge upon all who have mistreated him. He began keeping an extensive log of his detailed recollections of mistreatment. Eventually this drive went beyond passion to a feverish need to gain ever more power, so that one day all the multiverse, along with its petty and disinterested collection of gods, would kneel and weep tears of joy that they might be worthy to suffer under the horrific rule of Elderast.
Now, Elderast travels with whomever he believes will be most useful to him on his quest to gain power. He secures the loyalty of comrades by being useful to them as a wizard, but otherwise imagines that they will one day serve him in some capacity.
Elderast has an appearance that troubles weak stomachs. His hygene is poor, and his appearance is always crusty, with every crevice and wrinkle harboring little bits of matter that contribute to his pungent odor.
Always with him is the staff of his former employer from the college, who "disappeared" under mysterious circumstances. Atop the staff, Elderast has placed the desecrated lock of his mother's hair, still soaked in urine and caked in feces, encased in dark red amber as a constant reminder of the revenge that drives him onward. He also keeps a tome in his possession at all times that he calls "The Imperial Agenda", which he frequently updates with names of those he encounters or travels with, keeping detailed and constantly updated records of how they have slighted him in conversation and what their payment or punishment will involve once Elderast assumes control of the multiverse.
Elderast is largely treated as a senile old man, a reputation his strange, sometimes non-sensical ramblings have earned him. His threats are generally not taken seriously by those in his party, recognized as the empty babblings of an old man that they usually are.
In addition to his strange personality, Elderast seems to believe that the most powerful and darkest magic can be derived, not from the blood, but from other bodily products, such as his own mucus, urine and feces, which he frequently uses as additional spell components.
He never washes his hands.