Osteogenesis imperfecta (Riren)
Osteogenesis imperfecta, it is a disease that makes your bones weak, but it also something Eren Yeager has, he is 15 and lives with both of his parents. He doesn't let this disease bother him, but others at school do. His bullies, Levi, Erwin, Jean and Petra. The disease causes Eren to struggle breathing and he has to use an oxygen tank now and then.
One day when Eren is at school, he sits in his favorite area which is by the pond, but he doesn't realize Erwin and Jean coming behind him and Petra following them with a video camera. The two males throw Eren into the pond. Eren is panicking and doesn't know what to do and he's going under the water, drowning. Someone saves him, someone he didn't expect to save him, Levi Ackerman.
As the two get to know each other more, Levi learns secrets about Eren he didn't know, and they are to do with Isabel and Farlan, his two adoptive siblings that killed themselves.