Batman Gambit: Psmith's occasional modus operandi. In Psmith in the City, he stays in his job by cultivating a rapport with his supervisor, despite his occasional flagrant disregard of the rules.
Book Dumb: Gets expelled from Eton and sent to Sedleigh instead. Nonetheless an insightful and brilliant social manipulator.
Breakout Character: Introduced as a supporting character to Mike Jackson, but took over the series to the extent that Mike is now invariably remembered as Psmith's supporting character.
Carry a Big Stick: In Psmith, Journalist, Psmith uses Billy Windsor's walking stick (purchased specifically for such a situation) to fend off a group of New York City thugs. When one of the thugs shouts, "He's got a big stick!" Psmith mutters to himself, "I andRoosevelt."
Cloudcuckoolander: Psmith pretends to be one because it amuses him and throws people off their guard; nevertheless, he is always aware of and in control of the situation.
Comical Overreacting / The Comically Serious: Swings wildly between the latter and the former. (Death threats and angry mobs are nothing serious. Having to take public transport is a tragedy.)
Heterosexual Life-Partners: With Mike. Just to cement it, they end up marrying (respectively) Phyllis and Eve, themselves possible examples of this trope.
Hidden Heart of Gold: When Mike tries to thank him for sacrificing himself to save Mike from expulsion, he acts upset and pretends to have no idea what Mike is talking about.
High-School Hustler: Psmith's original role. Even after he leaves school, his interactions with authority figures retain some of the same style.
Hypocritical Humor: Psmith often refers to himself as "a man of few words".
My Favorite Shirt: His reaction to having his hat destroyed in Psmith, Journalist. He becomes determined to make the culprits pay...for the hat,and, in fact, succeeds.
Neat Freak: Often described as flicking minute specks of dust or lint from his clothing.
Standing Between the Enemies: Mike and Psmith contains an amusing subversion. Just as Mike and Adair are on the verge of beating each other senseless, Psmith stands between them and tells them not to fight...because they're in his room and they should be fighting outside. He proceeds to lead them outdoors and referree the battle.
Sudden Name Change: Originally named "Rupert", Psmith gets his first name changed to "Ronald" to enforce the One-Steve Limit in Leave It to Psmith.
We Help the Helpless: His newspaper advertisement in Leave it to Psmith states that he's willing to do anything for money, from walking dogs to commiting crimes, "provided it has nothing to do with fish".
Workaholic: Parodies this trope aggressively in Psmith in the City. He actually hates the job and spends most of his time cheerfully slacking off.
Book Dumb: Brilliant cricketer, not so good in school.
Chronic Hero Syndrome: In Psmith in the City Psmith accuses Mike of developing an addiction to sacrificing himself for others, likening it to "dram-drinking".
Class Clown: Was often in trouble for "ragging" back at Wrykyn.
The Confidant: One of Mike's duties as Psmith's secretary.
Demoted to Extra: In Psmith in the City he and Psmith are still on roughly equal footing in terms of page time, but by Psmith, Journalist he's become a side character. In Leave It To Psmith, he's a bit player who disappears quickly from the action.
Didn't Think This Through: Tends to make the quickest, most practical move required to save someone without fully realizing the consequences until it's too late.
Dumb Jock: Not dumb per se, but it's obvious he'd be doing better in school if it were not for his single-minded pursuit of athletic interests (namely, cricket). This is part of the reason his father sends him to Sedleigh, a school with less emphasis on athletics.
Early-Bird Cameo: Possibly. In Wodehouse's school story The White Feather, also set at Wrykyn, there's a minor character called Jackson (no forename given) who may be Mike or one of his brothers.
A couple of Wodehouse’s early school stories, “Jackson’s Extra” and “The Deserter,” feature a Jackson of Spence’s house, who is “the only Wrykinian who ever got three centuries in first-eleven matches and five extra lessons in the same term” or “who ever got a double century in a house match and four hundred lines in the same week.” This is consistent with the behavior that gets Mike sent to Sedleigh. The cricket captain at Wrykyn in these stories is Henfrey, whom Mike nearly replaces at the beginning of "Mike and Psmith," so it’s very likely that Jackson of Spence’s is an early appearance of Mike. Jackson in "The White Feather" is probably the same Jackson of Dexter’s who plays a small but crucial role in “Jackson’s Extra.”
Last-Name Basis: "Mike" in the narration, but "Jackson" (or "Comrade Jackson" to Psmith) in the dialogue. Boys at British boarding schools went by their surnames.
Word of God, he and Eve, eventually (get together; when the series ends, they're engaged).