This is the first Volume in the Light Novel series Invaders of the Rokujouma!? (Rokujouma no Shinryakusha!?).
Just starting high school, Koutarou is moving out on his own to take some of the burden off of his widower father. Lucky for him, he’s found a room at Corona House. It has a great landlord, it’s not too far from school, and best of all, it’s dirt cheap. It really is perfect... except for the strange girls that keep appearing to try and take it over! But even as room 106 turns into a battlefield, Koutarou isn’t willing to give up his apartment without a good fight. The invasion begins!
Koutarou Satomi moves into Room 106 at Corona House with help from his best friend Kenji Matsudaira. The cheap rent of 5000 yen per month attracts him to the room and he doesn't believe the landlord's (Shizuka Kasagi) claim that the room is haunted. As Koutarou goes to work the next day, he hears a voice calling out to him. Thinking it a prank from Kenji, Koutarou follows the voice into the woods and falls into a hole and hitting his head. The voice comes from an ancient statue, claiming that it has waited all this time for him and begins to glow. Koutarou blacks out and wakes up in the hospital, forgetting his encounter with the statue, with a worried Kenji saying he fell and hit his head at the work site. As Koutarou is being discharged from the hospital he comes across Harumi Sakuraba, causing him to recall their encounter a month earlier that led him to joining the knitting society. After speaking with Harumi, Koutarou heads home to Room 106 where he encounters a girl named Sanae Hoshiganan who tells him this is her room and that he must leave. Sanae reveals herself to be the ghost haunting room 106 and she begins to fight with Koutarou to chase him out of the room. after fighting throughout the night, during which he finds out charms negate her powers, Koutarou calls a truce so he can go to school. As Koutarou is going to school with Kenji and Shizuka, he mentions that the ghost of room 106 appeared. This worries Shizuka, but he assures her he is not going anywhere. Koutarou come back the night after his part time job and decides to talk with Sanae about who gets the room, however before the talk can get anywhere a girl comes crashing through the window on a broom. Sanae and Koutarou put their discussion on hold to take care of the unconscious girl. The girl eventually comes to naming herself Yurika Nijino, a magical girl sent to protect room 106 from evil magical girls. Koutarou and Sanae don't believe her and assume she is just a cosplayer that wants the room for parties. After throwing her out they are forced to let her back in when she begins to make a scene outside. When Sanae reveals herself to be a ghost, Yurika afraid of being possessed hides in the wardrobe and refuses to come out. After a night of trying to convince to come out, it becomes morning again and Koutarou goes to school. Heading home after club, he breaks up a fight between Sanae and Yurika, but before they can talk things out, a hole opens up in the floor and a girl comes out. The girl introduces herself as Kiriha Kurano, an underground dweller, and that she came to rebuild an altar for her ancestors at room 106. Koutarou hears her out and after bribing him with gold so that he can find a new place to live, he agrees to give her the room. Upon finding out however, that the altar will be used to make weapons to invade the surface with Koutarou refuses to give her the room. The four of them (Yurika, Sanae, Kiriha, and Koutarou) decide on a list of rules to settle the room's ownership by playing games on a point system. After playing games all night, no one comes out on top as they all have an equal amount of points. Koutarou then calls a hold on the games so that he can go to school.After school at the knitting society Koutarou falls asleep because of the previous three days lack of sleep. After waking up and returning to room 106 he finds the lights are out along with the 3 girls staring at a glowing circle on the wall. After examining the wall a young girl comes out of the wall with Koutarou accidentally touching her breasts. This along with everyone insulting her results in the girl starting a fight with them. After another girl appears and stops the young girl from continuing the fight (and destroying the planet) everyone decides to talk it out. The young girl is revealed to be Theamillis Gre Fortorthe, princess of a galactic space empire and the other one Ruthkania Nye Pardomshia, her bodyguard.After explaining that they need to claim the room and make Koutarou swear loyalty to pass a trial making her worthy for the throne, a fight breaks out between Theia, Sanae, and Kiriha over the room. This starts causing damage to the room, making Koutaou attempt to step in as he knows how much Corona House means to Shizuka. the fight gets further out of control, causing more damage to the room, until Shizuka shows up and goes berserk seeing the damage done to the room. She easily defeats everyone and after listening to their circumstances makes them decide on the room's owner peacefully. Despite everyone's reluctance Shizuka's power forces them to agree, thus signing the Corona Convention. At this point Koutarou passes out with a fever as his body could no longer hold out after 4 days of proper rest. After waking up and going to school, he remarks that school is the only time he can now feel at peace. The teacher then reveals that there are new transfer students (the invaders), surprising both Koutarou and Shizuka as they both know what is really going on.