Buizel was given to him by his adoptive father at his deathbed, as a means of thanking him for being the son that he could never have. Even prior to this, Buizel has always been a good friend of Ashton, playing with him constantly as a child.
Buizel appears to be a somewhat boastful pokémon. It regularly likes to show off, and thus, constantly prefers to be in Contest appeals rather than anything else, something Ashton actually reciprocates. Aside from this, Buizel is rather playful; it easily interacts with other people, as well as Ashton himself, and took very little time to be accustomed to him as its trainer, though they were acquainted with one another prior to this. Buizel also appears to be confident in its abilities, enough to rather brazenly challenge other pokémon and trainers, which was the case when Ashton inadvertently challenged Lily and her Sneasel.