Juunenme, Kikan wo Akirameta Tenisha wa Imasara Shujinkou ni Naru

In the midst of his daily life in Japan, Tohru was suddenly transferred to another world. In order to find a way back to Earth, he worked as a solo adventurer, and before he knew it, nine years had passed. Now in his tenth year in this other world, Tohru has finally resigned himself to living out his life here. That is when he receives a top-secret request from the head of the Adventurers' Guild, who is impressed by his abilities. The job is for him to protect a pair of smart and beautiful twins, Yuufi and Merry, from Harlan, the man in charge of the trading company they had inherited! Tohru decides to help the twins, and it will put his honed skills to the test...? His tenth year of living in another world is going to be a lively one.

(Source: MF Bunko, translated)