PERCY THINKS YOU ARE RAD! The rock thing is cool okay and you work it like a pro. The downsides of being made out of rock have probably not occurred to Seaweed Brain here yet so he's just like oh yup, that's a thing. If he actually thought about it for more than two seconds he might be a little more D: For no he definitely appreciates Santo's kind of light hearted attitude towards shift, although as he starts getting angrier/darker moods about his situation rn he might react a little more negatively because gd shit's serious bro. But ultimately he just likes the big guy c:

Katniss on the other hand is more wary. For a start, humour's not her thing so every time Santo opens his mouth she's just like 'uh :|'. On the other hand, Finnick seems to like/trust him, and Finnick's thoughts hold a lot of weight with Katniss. So basically while those two bromance around, Katniss is the one standing off to the side linefacing and thinking about what she'll do to Santo if he squishes Finnick or something.