This is Ritsuka Fujimaru from FGO, he's is a last master of humanity.

Humanity lost control of the seas to a mysterious and dark enemy called the Abyssal Fleet. They are mysterious, monstrous forces that suddenly appeared and took control of the world's oceans. Humanity's only defense is the Kanmusu which is against the Abyssal Fleet. Create your own Kanmusu, by what coming of your mind in headquarters, where you manage your resources, repair and upgrade your ship-girls, build new ships, and send out fleets for combat and supply gathering.

This is Made In Abyss RPG, (Spoiler Warning) where you can make the 8th layer (or not), you can make your character, gender, appearance, and personality. You can choose your own route to live or not. Remember try not to go up, it will effect your character do it at your own Risk. An Abyss is a colossal pit discovered 1,900 years ago around the islands of the southern ocean of Beoluska; a vertical hole with an opening diameter of around 1,000 meters and speculated to be over 20,000 meters deep. The deepest layer was unknown if you have a require to survive more than ever. (Or make it into a crossover of another anime). Made in Abyss is a dark fantasy-mystery set around a colossal, uncharted crater stretching miles into the earth. Filled with ancient, highly advanced technology, the hole operates on a brutal physics system where descending is perilous, but ascending triggers a physically devastating and fatal curse. At the bottom of the Abyss lie the ruins of an ancient, highly advanced civilization. Scattered throughout the pit are mysterious, powerful artifacts. Among these are autonomous, intelligent robots called "interference units," believed to have been created millennia ago by an entity living near the very bottom to act as scouting probes.The ascent curse operates as a one-way psychological and biological filter. It is directly tied to a force field that permeates the entire hole, which becomes increasingly harsh the further down you go. Now start. *Don't use the voice because its not any sense*

You were exploring the Yokai Mountain and somehow A curious shy but a genius engineer kappa girl named Nitori Kawashiro, shes seemed interested in humans.