All four members of Player Character can play guitar!!
Brian plays it in the STAMINA ending of Roadtrip. Amira does too but I got a different screenshot for her.
Oz and Amira play it in the original polaroid showing off Player Character. Oz also plays it in one of the animations for Monster Camp with his little phobias.
And Vicky plays it at the Picnic Spot! Unless I am mistaken and that is a ukulele. Eh, doesn't matter much, she's the lead vocalist.