Board Thread:General Discussion/@comment-26680716-20160501054031/@comment-26347196-20160509225634

I guess it can be both characters' theme? Because after all, it's a battle against Sans, where the story around the battle kinda revolves around Sans and Frisk/Chara.

To be honest, Chara is canonically not related at all to the battle against Sans. Fans, however, put Chara into the scenario, which, in turn, may be thematically correct, but not necessarily canonically true.