Microsoft Mike is a text-to-speech voice. He replaced Sam if he's unavailable. But that's not just his only purpose. He could be anything. From his brother, a friend, a nuisance, or even an enemy. His female counterpart is, arguably, Microsoft Mary.
One thing's for sure: He talks like a lady. Predictably, he denied the truth.
Biography
As with Sam, he has no actual bio. Depends on the author.