Raised in a poor farming family outside of Viridian City, Alister Sento spent his entire childhood seeking his stern father's approval. However, chubby, short-tempered, and whiny as he was, Alister was constantly upstaged by his far stronger and more cool-headed older brother. When the old man died, his will stated that a duel would decide which of the two siblings would recieve the family farm. Alister trained for weeks before the fight, believing that he would finally have his chance to prove himself over his brother. All told, the fight lasted two minutes: Alister's brother emerged with a new farm and Alister himself left with two broken ribs and a black eye. Disgraced, angry, and ashamed, Alister left his old life behind and set out to prove his worth and become a warrior stronger than any other.
Unfortunately, things continued to work against Alister. Only a few hours after setting out on his grand quest, he found a small Mankey wounded and unconscious in a clearing. Puzzled, Alister approached the wounded critter. It was covered with scratches and bite marks of a variety Alister recognized only a second too late. The pack of Rattata, having returned to eat its delicious Mankey prey, saw an even bigger meal in poor Alister. With a reckless ferocity the beasts overtook him, knocking him to the ground. It took a chunk of his left side and the intimidating shadow of a Fearow overhead to make the beasts move on. Once he was sure the threat was gone, Alister got up and began to scramble for the nearest town. However, as he left the clearing, he remembered the injured Mankey. Deciding the beast could make a refreshing snack or a quality decoy should the need arise, Alister turned, took it by its tail, and continued on his way. Luckily avoiding any other unfortunate encounters, the boy eventually stumbled upon the peaceful village of Pallet Town. With his senses fading fast and the blood seeping from his wound, Alister made his way to the most salient building in the town. He threw open the door, dropped the Mankey unceremoniously, and collapsed upon one Professor Oak's laboratory floor.
Alister was up to full strength in only a few days, but the Mankey's condition was far worse: several of its bones were broken and the poor creature could barely walk. Oak decided that Alister, whose medical care was costly and whose personality was perhaps even more taxing, would help to nurse the Mankey back to health as a means of repaying his debts. Although reluctant at first, Alister eventually began to enjoy his time with the Pokemon. At first, the two would go on walks around the lab together; as the Mankey's condition began to improve, the two engaged in more rigorous activity. They eventually became sparring partners of sorts, developing a deep connection through battle. After only a few months working together, the two shared a bond stronger than the sort Oak saw between most partners of several years. It was merely a formality, then, when Alister caught the Mankey as his own.