Minsc is a massively strong warrior from lands far to the east in Faerûn. He travelled to the area of Baldur’s Gate while upon his “dajemma” (a certain rite of passage) serving as the protector of the witch, Dynaheir.
While a dangerous berserker in battle, it appears that the ranger also suffered a head wound during his travels and has formed an extreme attachment to his hamster, Boo. Minsc believes Boo to be none other than a Miniature Giant Space Hamster who talks to him and serves as his supposedly-more-intelligent mentor.
Regardless, Minsc is a fiercely loyal companion who is rarely found far from his adventuring party.
Read more at IDW's Legend of Baldur's Gate.