Samgrass is the fly in the Oxford ointment. He’s like long-distance parenting for Sebastian on behalf of Lady Marchmain. In short, he spoils all the good alcoholic fun while the boys are away at school. But perhaps worst of all is Samgrass’s deluded idea that he is friends with Charles and Sebastian. Charles is not a happy camper after the Old Hundredth incident:
For the rest of that term he haunted us. Now that we were gated we could not spend our evenings together, and from nine o'clock onwards were alone and at Mr. Samgrass's mercy. Hardly an evening seemed to pass but he called on one or the other of us. He spoke of "our little escapade" as though he, too, had been in the cells, and had that bond with us.
Aside from being a complete buzz-kill ('90s slang of the day!), Samgrass is a great example of Lady Marchmain’s ability to essentially own people. The first time Charles asks, "Who is Mr. Samgrass of All Souls?", Sebastian’s immediate response is, "Just someone of Mummy’s." This proves to be very true. Lady Marchmain has Samgrass by the you-know-what, and though we never directly see her telling him what to do, it’s clear from his actions at Oxford that he’s operating strictly at her beck and call. What’s in it for Samgrass? As Charles says, "he was someone of almost everyone’s who possessed anything to attract him." Samgrass likes to schmooze with the wealthy and aristocratic, and Lady Marchmain is right up his alley.
Now, one last interesting point. As it turns out, Waugh based the character of Mr. Samgrass on a professor of his at Oxford named Maurice Bowra. Looks like the bonus in writing a novel is getting to satirize all those profs you didn’t like in college.