After a point in the VN when it shows Asterion with this sprite, I can't not see him holding a guitar. Maybe he plays Argos' theme? Maybe he's a fan of Nickelback? The possibilities are endless.

Was this sprite used with that in mind in another time or another story?

I'm hoping there are more sprites added for his relaxed speaking parts. 
This is part silly, part feedback.

This picture was taken from Google. I added the guitar.

A lyre is just a very early guitar, if you think about it.

it's a little difficult to come up with inventive neutral resting poses. there are only so many characters you can have their arms crossed by default before it starts looking like you are drawing the same character over and over, just with a different skin.