Otherworldly Warrior Training Diary: From Frail High School Student to World's Strongest

Kyoya Kusunoki had been living an ordinary life with his excellent older brother Touya, his tomboyish but physically weak younger sister Saya, and his taciturn father Toshinori. However, he is betrayed by Akane Kogetsu, his brother's fiancée and childhood friend. His brother goes missing, and his father dies suddenly from stress. He tries to live quietly with his younger sister Saya, but he is forced to accept a condition imposed on him by Ryugen, the father of Akane Kogetsu, as a joke. The condition is to rank first in his graduating class at the high school where Ryugen serves as principal. The Kogetsu family is one of the world's leading financial conglomerates and holds overwhelming power and influence in Japan. When hints are made about Saya's future, he has no choice but to accept. Thus begins his school life, but he experiences severe bullying from students and teachers on a daily basis. Finally, when he is told he must withdraw from a "martial arts" practical training and is at a loss, he discovers a gate in the basement of his home. This gate connects the parallel world Arius to Earth. Kyoya resolves to train in this parallel world Arius and steps through the gate of adventure.