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.
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.
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.
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.
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.