Katniss is tentatively convinced that he's not a mutt, because she's still alive and he hasn't tried to murder her (good job restraining yourself, Santo). I think for now he still creeps her out a little, but she'll probably stalk his interactions with the other THG babbies and warm up eventually. As much as Katniss ever warms up, which mostly just extends to insults that she doesn't mean, as opposed to insults that she does.

Percy thinks that Santo is rad as hell. Dude's made out of rocks, man! And doesn't want to kill him, even better! He also appreciates that they managed to have a sort of semi-serious conversation outside of the screwing around with turtle jokes, haha. Much as Percy can goof off, he's not a total dork, and he's concerned about what's going on with the turtle too, so throwing ideas around with Santo is/was something he's glad to have had.