Kage no Jitsuryokusha ni Naritakute! Master of Garden ~Seven Yin Retsuden~

Not the main character, and not the last boss. The one who lurks in the shadows of the storyline and holds all the power, hiding his true abilities by acting like a side character. The boy who longs to be a ’Shadowbroker’ lives an unobtrusive life as a side character, training to gain magic power until he’s killed in an accident and reincarnated into another world.