Kyoukai Meikyuu to Ikai no Majutsushi

Kyoukai Meikyuu to Ikai no Majutsushi (境界迷宮と異界の魔術師, lit. "Boundary Labyrinth and Magician of Alien World") is a light novel series by Eiji Onosaki with illustrations by Tetsuhiro Nabeshima.

Plot
When Theodore was dropped into a waterway by his half-brother, he suddenly recalled the memory of his past life.

He had been a Japanese citizen known as Kirishima Kagehisa, placed inside VRMMO character he had made in Break Force Online.

"Theodore" is a Combat Mage that Kagehisa played within BFO, and he realized his skills were progressing well even in this other world.

Theodore, who had lightly brushed off his half-brother to make his own place in the world, had ventured off to the large labyrinth existing at the Boundary City, Termuilles.