Basil of Baker Street, also known as Mr Basil, is a male sentient mouse.
Basil lives under the appartment of human detective Sherlock Holmes, from whom he has picked up tricks and mannerisms. Like his human "neighbour", Basil is the most famous detective of his society, astoundingly clever and very knowledgeable in chemistry.
Basil is, like his archnemesis Ratigan, extremely intelligent and knowledgeable. He is solitary and somewhat grumpy, as well as highly megalomaniac, but is nonetheless firmly on the good side; much of his hermit-like persona seems constructed, and bits of unrestrainged selflessness sometimes shine through (such as his initial shock when learning that Olivia Flaversham was an orphan).
Basil is a slender mouse with a long muzzle and light brown fur all over his body; he has a very thin black tail.
Basil has had a long career in the investigation business, becoming extremely famous, especially for finding abducted people. Most of his work seems to have aimed toward arresting Professor Padraic Ratigan, until the events of 1897, when, in spite of the rat's escape, Basil still mostly won over Ratigan and began to investigate non-Ratigan-related cases.
According to the unofficial short story, Basil lived with his parents until the age of ten. During his early years, he had begun to develop a liking for readings, and, oddly, preferred academic books to fiction. He spent most of his time in the school library, solitarily studying, and rarely if ever socialized with the other children, whom he quickly found boring, with the exception of a girl named Esther. When he turned ten, he was sent to a boarding school, where he did extremely well even though he didn't study much, thanks to his innate talents, but was often mocked.
Three years later, when a girl had her diary stolen, and that an obviously innocent classmate of hers was framed for the theft, Basil took it upon himself to right the wrong, as much for justice's sake as to prove himself he could solve a criminal case; he uncovered the real culprit, solving his first case. That same year, he took an interest in chemistry, becoming extremely talented; his skill was noticed by his teacher, who advised him to pursue a career as a chemist, and therefore study the subject at Rodent Oxford.
During his second semester there, he met chemistry teacher Professor Padraic Ratigan, who quickly became his only friend and his mentor. Recognizing Basil's intelligence, Ratigan offered him a position as his lab assistant. This unfortunately led to Basil discovering notes, made by Ratigan, about a dreadful project of his to drug the whole city of London into stupidity to be able to rule over them. Basil confronted his teacher, who quickly revealed his true colors, and offered Basil to become his partner in crime, arguing that he was just as much of an outcast as he was, and that them both, only, because of their intelligence, deserved to rule. Basil immediately refused and denounced Ratigan, who, thanks to the ludicrous scale of the plan, was able to convingly plead madness, and quickly escaped the asylum where he'd been placed, sending Basil a telegram that notified him that their game wasn't over. Basil abandonned his studies and became a private detective, spending most of his time investigating cases related to Ratigan's hidden actions.