The Eminence in Shadow, Vol. 1 (manga) — Daisuke Aizawa

Cid Kagenou has a dream. Not of being some typical protagonist or the Final Boss -- he has his eyes set on becoming a hidden mastermind working in the shadows! Now that he's been reborn in another world, Cid has been hard at work building the perfect stage to act out his long-desired role. The only issue? His imaginary adversaries and plot devices seem to actually exist in this new realm…and he alone is left in the dark!