Goldenfishwolf asked
"Oh, I also curious about the "battle theme". I heard that some monster will have their own battle theme when they trigger a fight. Is that true? Moreover, do monsters compose their theme by themselves?"
Frisk:Yeah, that's actually true! Some of them made their own themes, such as Undyne and Mettaton, but a good majority of them were actually made by Toriel and Asgore.