Stephen thinks the Pokemon Trainer from Brawl is the worst Pokemon trainer ever, because he can only capture statues and, when their play his Subspace doppelganger, his Charizard jumps off the edge and kills itself in seconds.
Crystal from Crystal's Pony Tale captures the other ponies' souls.
Beware the Nice Ones: Stephen comments on how many of the fights Argyl gets into should just be talked out, right before killing everyone.
Beware the Silly Ones: Faendal may be an idiot sometimes but he can be terrifying as demonstrated when he thanks Argyl for killing Sven, and especially in "Skyrim #181" when he murdered Argyl.
Book Ends: He feels this way during the Dark Brotherhood questline which started with him leaving an abandoned shack and was about to end with him fast traveling back to the shack to catch the boat that has his final target, The Emperor.
Born Lucky: Mal, sometimes - for instance guessing the safe route across a dangerous floor in Gears of War the first time after not playing the game for years.
Bumbling Sidekick: Faendal tends to make things either silly or difficult for Stephen, sometimes both.
Butt Monkey: Faendal in Skyrim. Carl Sagan the Pidgeotto gets this in Pokémon LeafGreen to some extent too.
Captain Obvious: Mal in Brawl. A Running Gag is her non sequiturishly responding to something Stephen says by announcing what character she's playing as or what item she's holding.
An aversion in Shadow of the Colossus: he gets very frustrated with Celosia, and after a grueling battle releases a tirade of (bleeped out) swears on the poor creature. At the end here.
Completely averted in The Last of Us: when reading the different document artifacts he finds in the world, he reads out the swears as well. Justified, as censoring them in a game like The Last of Us would be rather pointless.
Hypocritical Humor: Faendal kills Argyl for killing an NPC but after a reload goes and kills the exact same NPC.
In the first Mario Kart race between Stephen and Mal, Stephen tells Mal not to get too cocky because it's only the first lap of the first race. Almost immediately after, and debatably through the entire LP, Stephen ends up getting insanely cocky, usually just before a humiliating fall.
Improv: Stephen is known for being able to improvise songs about what's happening in a game on the spot, sometimes set to the game's music but more often based on the tune of a randomly chosen real song.
Madness Mantra: "Bait...rock...ball. Bait...rock...ball" in Pokémon LeafGreen's Safari Zone.
"Gimme something good, gimme something good, gimme something good!" when Stephen hits an item box in Mario Kart and is waiting for the roulette to settle on an item.
Mature Rating: At the opening of each video for "Grand Theft Auto V", Stephen shows the rating screen, usually with added humor that relates to the episode about to be shown. (Rating the video 'I' for Immature, etc)
Technical Pacifist: Stephen sometimes acts like this in Skyrim especially during the Dark Brotherhood questline, such as having Lil' Brekflek murder his targets, untill Brekflek went of a murderous rampage, causing Stephen to kill the rest of the targets himself, which even after that he still tries to get him to attack him first so that he kills them "in self defense".
Third-Person Person: Stephen when playing as "Cold Kitty" in episode 9 of Battle Block Theater.
The Unfavourite: Stephen loves Donkey Kong Country, but hates playing as Donkey Kong in it (as opposed to Diddy Kong) and repeatedly complains about how inferior a player character he considers him to be.
Verbal Tic: Mal's distinctive "Uh-huh" when Stephen's asking her questions about a game.