Aspen's voice need to sound kind but weak and a little shy.  Later in the comic he suddenly becomes taller so it would be nice if he can have a little bit of a more mature voice when that happens, but still shy.  (this can also be played by two people for when he is small and when he grows)

Aspen maybe the answer to breaking Ember's curse but he doesn't know it.  He has the opposite problem of Ember where he is always too cold and because of this Aspen was never able to grow taller. Aspen is super kind, maybe too kind, and he always thinks of others and how they might feel.