Here is who I think would best fit each role and why. These are based more on personality than appearance. (Possible spoilers for Undertale and Ouran High School Club below!!)
1. Haruhi- Frisk: Frisk falls into the underground and Haruhi stumbles upon the host club, and both are thrust into the excitement of it all. Haruhi is mistaken for a boy and must become a host through extenuating circumstance. Frisk is ambiguous throughout the game and is attacked by the monsters in the underground. In the manga and anime, most of the characters develop feelings for Haruhi and flirt with her quite a bit. Frisk goes on a date with almost every major ch