Wrote this up aaaaages ago, never bothered to post it (but I posted Rena's. Go figure). Anyways, here goes.
For lack of a better term, Ban’s childhood was quite “interesting.” Not much is shared about it in the animé, and the manga gives information pretty sparingly as well, so this is a blend of what is given in both, along with some head-canon to fill in the blanks.
He was born in Japan - right in Shinjuku, ironically enough - to mixed parentage; his father was German (and referred to henceforth as Der Kaiser), while his mother is presumably Japanese; Ban is a quarter German. His name was initially to be Yosaku - a term meaning “a gift from God;” however, when he was still a baby, he kicked Paul Wan (now the owner of the Honky Tonk) hard enough that he called the child “Ban,” which became his name (“Ban” is part of the verb “yaban,” meaning “barbaric.” So in a sense, both names fit him).
While still a baby, Ban was brought back to Germany by Der Kaiser to be raised by his mother (Ban’s grandmother), the Witch Queen. It was while living with his grandmother that Ban learned of his abilities; the Jagan - also known as the Evil Eye, once his eyes meet that of an other, he can send the other person into essentially a living dream. It lasts for one minute in reality, but it’s up to Ban just what the person sees/experiences and how long that minute lasts. He also learns of the limitations of the Jagan, and the origins of his strength - having been born under the sign of Asclepius, he has inherited its curse; when it is invoked via incantation, he gains strength far superior to what he already possesses, and a coiled monster claw will form over his arm and hand (as seen in the manga). Also, he is taught in the ways of witchcraft by both his grandmother and her disciple, Maria Noches (the latter a manga-only character).
Seeing this for herself however, Ban’s mother disowns him as a demon*, and Ban is left to be raised by his grandmother until her death. Upon hearing of the Witch Queen’s passing, the Shamans - archenemies of the Witch Clan - begin a manhunt for her last known living descendant, as Der Kaiser has mysteriously vanished.**
From that moment, Ban lives on the run, staying all over Europe, but never having a permanent home. The last time he was in Germany, he managed to befriend the Miroku brothers (they suffered a falling-out later on, culminating in battles in the Venus de Milo arc in the animé and a fight that pitted all seven brothers against Ban in the final arc of the manga). He finally left Europe when he was in his later teens (16-18 years old), heading overseas to Japan to try and lose the Shamans. He has yet to run into any of them, and in Japan he lived first as a plunderer and thief. During this period, he ran into Takuma Fudou, another plunderer. In their final confrontation, Ban fought him and ended up ripping Fudou’s right arm off; since then, Fudou has had a vendetta against Ban, and wants nothing more than to tear Ban to shreds (and that’s putting it the lightest way possible). In a sense, Fudou does get what he wants; in the final arc of the animé, Ban and Fudou have one last confrontation, and though Ban comes out victorious, he uses the Jagan to give Fudou a vision of him beating the hell out of the younger man before he (Fudou) dies from his injuries.
Eventually, he befriends Himiko Kudo and her brother Yamato. The three of them had a close friendship, teaming up to use their abilities (Himiko is known as Lady Poison, and learned her craft from her big brother Yamato) in a thievery ring. That only lasted for six months though; Yamato shared his and Himiko’s history with Ban, telling him about the Last Children and the curse that had befallen himself and Himiko; although he thought they’d escaped soon enough, the curse was still within the siblings, and Yamato warned Ban that it was only a matter of time before he would be overtaken. After telling him not to tell Himiko any of this until it was the right time, he made Ban swear to kill him if the curse befell him; startled, but still not quite believing it, Ban agreed. It wasn’t long after that, while Himiko was out, Yamato began to lose his sanity; though Ban went to get help, Yamato instead begged Ban to kill him, asking him with his dying breaths to kill him while he was still human and to watch over Himiko. This is the scene Himiko chances upon; seeing her brother collapsed on the ground, chest soaked in blood, and Ban standing over him with a forlorn gaze, his hand covered in Yamato’s blood. He left Himiko without a word, hearing her own vows to kill Ban in revenge for him murdering her brother, his only memento being Yamato’s lighter, which he carries to this day.***
Ban lived on the streets of Shinjuku for awhile afterward, having to fight to survive. A jaunt into the Limitless Fortress brought him face-to-face with Ginji in his Raitei (Lightning Emperor) form; the two fought, Ban barely defeating him, and when Raitei regressed back to Ginji, Ban left him with the words “If you don’t like it here, then why don’t you just leave?” It was common sense for Ban, but much more difficult for Ginji to comprehend, as the Limitless Fortress was, in all senses of the term, his home; Ban has pretty much been a vagabond for most of his own life.
It’s not long afterward that both Ban and Ginji are approached by the second generation GetBackers (apparently a trio, as it looks in flashbacks); they’re given the title, job, and told to go to the Honky Tonk. Thus, the place has become their hangout (and in DDD canon, Paul’s finally allowed them to live in a loft above the café, which is where Ban and Rider now live, with Ban and Ginji paying off their massive tab in lieu of rent) and the place where people come to hire the GetBackers.
The life of the GetBackers has been Ban’s life for the past two-three years; it’s the most stability he’s had since he was a young child, and it’s the happiest he’s been.
* ep. 12; when Ban has invoked Asclepius and is attacking Ayame in his body modification lab, Ayame turns fearfully to Ban after having the spikes torn off of his body and whispers, “You demon…” Holding one of Ayame’s spikes in his hand and preparing to stab the other man, Ban gazes at his reflection and remembers his mother’s words: “This…this isn’t my child…this is a demon child…!” Rather than go through with his original intent (stabbing Ayame), Ban instead smashes the mirror with the horn and leaves Ayame a blubbering mess while freeing the people he had been intending to use as guinea pigs in his body modification experiments. As such, one way to get on Ban’s bad side immediately is to call him a demon or monster.
** Der Kaiser in fact shows up again in the manga; in the final arc, he confronts and fights Paul Wan, who was Der Kaiser’s close friend and a member of the second-generation Get Backers (in the animé, Paul used to the be the negotiator in a thievery ring), and then has a showdown with his son. It ends when Ban manages to hold back from using Asclepius to kill his father, and instead uses his own strength to fight, forming the Angel Wing and non-mortally wounding him. It’s then that Der Kaiser surrenders, and before leaving his son again, asks Ban for a light for his cigarette.
*** Apparently, Ban and Himiko are somehow related - in the manga, it’s stated that they’re siblings through consanguinity, but never stated nor addressed in the animé. As such, I’ve decided to go with the idea that they’re distant cousins; though Ban does regard Himiko as a sort of younger sister at times, he’s unaware of any actual relation.
Yes, this could've been much longer, and some things may be added/deleted/retconned.