Sekai Saikyou no Kouei: Meikyuukoku no Shinjin Tansakusha

Workaholic assistant Atobe Arihito fell asleep on his company's bus while on a vacation. When he awoke he learned he had died due to the bus getting into an accident, and that he had reincarnated into a world of reincarnators—to the Labyrinth Country. To his surprise, he was discovered by his slave-driving manager, Igarashi Kyouka, who further explains their role in this world.

In Labyrinth Country, reincarnators are expected to choose a job and form parties to fight against monsters in Labyrinths. Kyouka had expected Arihito to work with her once again, but he declines due to his knowledge of her demanding nature. As they depart she vows to make him regret not choosing her. And thus starts Arihito's adventure as a "Rearguard."

(Source: Novel Updates)