Ruska

A young boy who has spent his entire life labouring in his village on a layer of the Vertical World.
Hardworking and earnest, Ruska is plauged by an insatiable desire to know what lies at the bottom of his world. After witnessing a fallen maiden (Omega) fall through his layer, takes a chance and plunges in headfirst after her. Sending him on a joruney that will reveal to him the truth of his and his world's existence.

It is revealed later on that Ruska is an Earthling from outside the Vertical World's universe, who escaped from a doomed planet Earth on-board a colony spaceship. Noticing an abnormality during the construction of the Vertical World, he ventured inside to investigate, but ended up losing all memory of his prior life as a consequence.