you are a human-(skeleton) moster hybrid living on the surface (after the true pacifist ending) after escaping from an abusive orphanage and living on the streets for around a year, you run into two skeletons...
{THERE IS LEMON BUT READING IT IS OPTIONAL AND WONT AFFECT THE STORY}
You've aspired to be a Monster specialist ever since the monsters had left the Underground. You leave on your first expedition to find as much information on each Monster as you could but then you're sidetracked when you wander into a cave, where the barrier once stood, though you didn't know it and helplessly wander into the Underground and meet a strange skeleton who really sparks your interest.