To be fair, Karlach talking to herself feels more natural to her character than it would for a lot of the other origin characters. Hard to imagine either Shadowheart or Lae'zel engaging in much self-reflection, and if Wyll tried it he'd probably wind up berating himself for making a deal with a devil.
Astarion might go in for a nice brood if he can find somewhere sufficiently dramatic to do so.
Gale, on the other hand, might regularly summon an illusory simulacrum of himself and engage in actual conversations, and that I'd seriously like to see.