As Kuja gives Frisk their part, there's clearly some mental re-arranging going on behind their eyes. Channeling their feelings for Lilac and Papyrus into some kind of sibling bond could work. The pain that hit them when Lilac left would be perfect for this scene too.

Frisk's demeanor changes. Their expression brightens from a cocky grin to a hopeful, eager smile. "Brother, oh my goodness you're-- wait." Their expression becomes confused, edging on worried. "Cassius? Is- is that... Where is he?" Their voice skirts dangerously close to panic as they finish their line.