K from Blip. All the fucking time. Surprisingly averted by the fucking Devil.
Though he is a rather nice guy when he isn't trying to kill everybody.
Alex Williams, aka Captain SNES: The Game Masta, unleashes one that apparently would have put George Carlin to shame. Mind you, he's not exactly a boy scout with his language anywhere else... this is simply him cranking his jackass levels Up to Immeasurable Levels.
Skoll more than the others, but nearly all the characters of Cry 'Havoc' are battle hardened and jaded mercenaries, so the language is not unexpected.
An Exterminatus Now strip has Lothar getting even more sweary than usual after falling off the back of an armoured car.
Lothar: Shit... cock... motherfuck-k-k...-unt. Either Virus or Eastwood: You screwed it up! What do we do now? Lothar: I just FELL OFF a moving vehicle and bounced down half a mile of tarmac, but I'm FINE, THANKS FOR FUCKING WELL ASKING!
Gamzee, one of the Trolls in Homestuck, uses various permutations of "motherfucking" in nearly every other sentence. Take for example his introduction page.
Dave, the 'coolkid', lays the fuckword on heavy. And Vriska's language is as blue as her blood.
Karkat manages to work swearwords into most of his sentences. It seems to be a habit born of perpetual crabbiness.
Muh Phoenix: Everybody at some point, but the Phoenix Force speaks like this constantly.
The Order of the Stick: Roy, of all people, loses his cool when he learns Xykon has left Kraagor's Tomb, ruining the Order's ambush plan.
Outsider: Talon lets loose a furious tirade against Clearbrook's comm officer when said officer tells her that their rendezvous is being broken off due to an incoming Umiak force and insults her when she objects. This is referenced a few pages later when Jardin refers to her as "Arrir Pottymouth".
. Mind you, he's not exactly a boy scout with his language anywhere else... this is simply him cranking his jackass levels Up to Immeasurable Levels.