Samgrass is Lady Marchmain’s way of keeping an eye on her son while he’s at Oxford. He’s also helping her out with her memorial book about her brother Ned. According to Charles, he spends his life sifting through archived documents and knowing everything there is to know about aristocratic families. Charles finds him to be very different from Lady Marchmain.
When Sebastian is on trial for drunk driving, Samgrass gives a character testimonial in his favor that is probably what keeps him out of jail.
Unfortunately for Sebastian, this fiasco leaves Samgrass thinking that they’re buddies, even while he’s disciplining him and Charles.
While talking of his time at Brideshead, Samgrass mentions Boy’s sister, Celia.
Mr. Samgrass tells Charles not to plan on boarding with Sebastian at Oxford, since Lady Marchmain wants him to live with Monsignor Bell.
When Lady Marchmain pulls Sebastian from Oxford, she sends him to stay with his father in Venice and then travel around Europe with Mr. Samgrass.
Samgrass brings Sebastian to Brideshead two days after Christmas. He pretends that everything went fine on their trip, but Charles knows something is up.
It finally comes out: Samgrass lost Sebastian, who went off on an unchaperoned drinking binge.
After he leaves Brideshead himself, Charles finds out that Samgrass was outed by Julia as a fake and left the estate as well.