When she was first introduced, Grim Eyes from Digger frequently rolled her r's. She does it much less frequently post Heel-Face Turn. Some fans have theorized the rolling was due to a growl. Honored Murai always refers to everyone as Honored Such-And-Such, which looks a bit odd when she mentions several names in a row... especially given her companions', uh, informal approach. Presumably it's cultural.
All the protagonists of Homestuck have a specific typing quirk when chatting online (which is carried over to real world conversations as well), usually tied to their personality; John uses no capital letters, Rose writes in an overtly correct manner, Dave eschews both capital letters and punctuation, Jade multiplies her punctuation and uses lots of smileys.
The trolls use more complex ones: Karkat types in all caps; Sollux (obsessed with doubles and bifurcation) types 2's instead of s's and "ii"'s instead of i's; the shy Tavros tYPES IN REVERSE CAPS LOCK and uses commas rather than periods; Feferi replaces h's with brackets )( (referencing her Zodiac sign, Pisces, and the shape of her horns) and precedes capital E's with a hyphen to make them look like her weapon of choice; and so on.
Curiously, their typing quirks also represent the way they speak - for example, Kanaya (who begins each word with a capital letter and uses no punctuation) is stated to enunciate each word carefully when speaking, just like she types; Sollux has a lisp; and so on.
Furthermore, the quirks make their way into sound effects - for instance, when Tavros uses his communing superpower to Mind Control animals, the sound effect is "cOMMUNE,"
More traditional tics: Aradiasprite's "ribbit", Gamzee's "honk", and Feferi's "glub". Even though they're actually typing.
Vriska's quirk – typing some letters and punctuation eight times for emphasis and replacing Bs and "ate" sounds with 8 – seems to be contagious, since John has picked up on the former, and hereditary, since Marquise Mindfang did the latter.
It also infects John's sound effects, with the Fluoride Octet-enabled late-stage Warhammer of Zillyhoo making a "8ONK" sound.
Jade later picks up another verbal tic in the form of going "woof" when excited or making other involuntary noises like laughter, after she attains Dog Tier.
The Killotron robots in Skin Horse no longer want to Destroy All Humans!, but they still use the word "destroy" in place of random verbs. It's considered a bit intimidating in-universe.
Unichat has the AI @hena, who compulsively spams the letter "G" in IMs due to a glitch.