The main protagonist of the story, Koutarou moves out of his dad's house so he can live on his own during high-school life. He doesn't have the greatest budget though, with only 5000 yen per month. Surprisingly, he finds a six tatami mat apartment that he can afford! Unbeknownst to him, the price was so low because of rumors that it was haunted! He then meets a Sanae, a ghost, Yurika, a "Magical Girl," Kiriha, an "earth person" and Theiamillia/Thei, an alien, and they all turn his life upside down in a battle to claim apartment 106 as theirs!

Thei often calls him "Neanderthal."

It is later revealed that he is the real "Blue Knight."