I understand that it is most likely just an OC, but nevertheless, it is needed. The fact is that Frisk in this Universe is a secondary, but key character.

In the history of HarmonyTale, Avril is the main one. She ends up in the Underground by mistake, trying to protect her brother from bullies (yes, how interesting). She travels through the Underground trying to find a way out to the Surface. But as she progresses, she reveals many secrets of this place, learns a lot about the people who lived here before her arrival. Thanks to her soul (Soul of Friendliness), she tries to help the monsters and restore faith in the future .... But can she? .... I don't know, until it's clear.