Not really being smart, Dark Raph often talks in bad English and is more of a representation of someone who is all brawn with no brains. He doesn't really think about what he's saying so often that he says sentences that aren't grammatically correct (e.g. "I'm gonna make you not move no more" from Bad Blood)
Acting upon rage like the actual Raphael, he often attacks violently, trying to slash people with his Twin Talon Knuckles like Raph would with his sais. When he is first seen, he wanted to crack Raph open to eat him in a sort of cannibalistic manner, but this could be also due to the fact food is scarce for them as Dark Leo suggested in DNA is Thicker than Water.
Raph also often suggests that Dark Raph has bad breath during their fights, often angering the latter more.
Besides this, he seems to be very concerned about his image seeing as he takes offense to any mention of his appearance. Mikey uses this as a distraction in The Fall of Darius Dun and Dark Raph is knocked unconscious believing that he is pretty. The other clones appear to know of this trait as well, Dark Mike telling Darius Dun not to insult his looks at the end of the same episode. Dark Raph thinks about changing at certain points in his life.