With the duet, it was somewhat more obvious that Frisk wanted them to join in - here it never even occured to them as an option, even leaving aside personal space distance.

Honestly, that Frisk can play any songs at all is a source of amazement to Chara - it shows in their expression. They're not just listening politely, or because that's the thing to do.

Frisk begins their second song, and they listen as raptly as they feel they can get away with. There's a lot of skill here, they think. They lose themself in the music a little, and once more start humming with it.

It feels... they don't know how to put it. Dark. Mysterious. They wonder who the important person was, and they wait until the end of the song to ask.