Ben was the mortal prison for deposed hell-goddess Glorificus. Over time, she learned to free herself from Ben for periods of time in order to search for the key. A spell prevented any human witnessing or hearing of this transformation from remembering it.
As a handsome junior physician at Sunnydale Memorial, he met and befriended Buffy Summers during her mother's illness, and unsuccessfully attempted to romance her. Buffy decided to focus on her family issues and not pursue him, not wanting to define herself by her lack of a relationship. He was an essentially decent and compassionate person, protecting Dawn Summers from Glory when he learned that Dawn was the key that Glory sought.
He summoned an alien Queller demon to Sunnydale in order to kill mental patients and "clean up Glory's mess."
As Glory spent more time in control of their shared body, Ben's life started to fall apart. He was dismissed from his job at Sunnydale Memorial because of increased time (as Glory) spent away from work. As their personalities begin to merge and the two were able to communicate directly, Ben agreed, out of self-preservation, to help Glory kill Dawn. However, Buffy defeated Glory by using a bulldozer and beating her to a pulp with the Hammer of Olaf. After being beaten, Glory transformed to Ben one final time, and because he was human, Buffy spares his life. She told him to warn Glory not to come near her and her family again. Quickly afterward, Ben was suffocated to death by Rupert Giles, who considered Ben an innocent for the most part, but was determined to prevent Glory from ever returning. His exact age at the time of his death was 25.