The other protagonist of the musical (and the one who it's named after), Chara is a bit snarky and at first mistrustful, helping Frisk out of obligation since they're stuck together,  She really opens up to Frisk over time however, and begins to see her almost as a sibling.

Also, since she and Frisk are main characters and thus there's a good amount of interaction, be prepared to sing a few songs. After all, the musical has her namesake—or rather, role-sake.  As Chara is in a large percentage of the songs, a wide range is good, and be aware that there is one song in particular that Chara gets that is pretty high. So keep that in mind when auditioning!