The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat, Vol. 2 (manga) — Rui Tsukiyo

The great assassin's next target is…the cosmetics industry!
After completing his assassination training, Lugh embarks on his last test as heir to the Tuatha Dé clan: taking on the business world. Now, he dons the identity of Illig Balor and forges a new path and connections as the illegitimate son of a tycoon. But will this final trial be enough to prepare Lugh for his most difficult assignment yet?!