Thomas Bell is Gregory House's new stepfather. Bell was an unseen and otherwise unnamed character who House started to believe at the age of 12 was his biological father. He was finally seen and introduced by name in the Season 8 episode Love is Blind.
At the age of 12, House started to become suspicious that John House was not his biological father as John was away when House was conceived and Gregory and John do not share several genetic features, particularly the relative size of their second toe. On the other hand Bell, a Unitarian minister and close family friend shares a birthmark with House. House was so certain of the relationship that he challenged his father about it the summer after Gregory turned 12. John didn't deny it, but stopped speaking to Gregory for three months.
When House attends his father's funeral in Birthmarks, he shares with James Wilson for the first time his suspicions about his paternity and points out Bell to Wilson. Wilson thinks House is just externalizing his dislike of his father and points out that Bell looks a little like Sean Connery. He thinks House is lying to himself about it because it comforted him to think his father was someone other than John House.
However, at the funeral, House obtains a DNA sample from John and later performs a paternity test. To Wilson's surprise, but not House's, the sample doesn't match, ruling out John as Gregory's biological father.
In Private Lives, Wilson is trying to get something embarrassing on House and Robert Chase notices that House's copy of "The Golden Bowl" is too thin. They find the book House has been reading and find it's really a book called "Step by Step Sermons for Everyday Life". They are both astounded that House is reading something religious.
They both decide to investigate. Chase strikes out by getting in touch with the author to order several dozen copies of the book. When House finds out, he wonders if Chase mentioned House's name, but Chase didn't and wonders why House cares. When Wilson sees the book with its original dust jacket, which has a picture of Bell, he recognizes him as the same man House pointed out at his father's funeral.
Wilson confronts him and House admits he was trying to find out more about the man. When Wilson asks why he didn't just telephone him, House says he didn't want to find out quite that much. House admits he learned nothing from the book, saying that behind the God stuff, there was just "more God stuff".
In Love is Blind, Blythe House comes to Princeton to tell her son that she is going to marry Bell. However, House tries to dodge meeting her. In order to get House to reach out to her, she makes Wilson think she has cancer by leaving misleading clues (like a book on living with cancer). House finally decides to see his mother at her hotel but, because he didn't call before coming, he interrupts her while she and Bell are having sex. As House barges in, Bell tries to hide, but is unsuccessful. Blythe announces their engagement shortly thereafter. Later, they admit they married shortly after John House's funeral.
Eventually, House confronts his mother and Bell about his paternity and Blythe has to reveal the truth to both of them. However, it appears that Bell was totally unaware that he was House's father and, in fact, is rather upset that the "pill popping, lunatic, sociopath" is his son. However, they soon reconcile and both men come to common ground over their love and respect for House's mother.
However, given that Bell is a kindly man with a great deal of compassion, Wilson steals a fork at dinner and runs another paternity test. He finds out Bell is ruled out as House's father as well. House confronts him about stealing the fork and realizes Wilson's curiosity got the best of him. He also realizes that if Bell was a match, Wilson would have told him and realizes that Bell wasn't his biological father either. Wilson confirms this by showing House the results. House begins to realize his mother is more interesting than he thought.