Lee Seob's Love

In his entire life, Lee Seop pursued to become a capable heir of the number one firm in Korea, TK Group. But his efforts were in jeopardy when a genius man manages to take control of the core business in the firm. Frustrated with his own incapabilities, he planned to retaliate in a nonsensical way. Meanwhile, Mikyeong, a top elite in the firm was assigned to work with him.

Note: This series is the spin-off sequel to the novel, 태연한 거짓말 which featured Junseop (Iseop's cousin) as the protagonist.

From Webtoon:
With her outstanding business acumen, Mingyeong Kang climbed the corporate ranks of TK Group, Korea’s top conglomerate, to become a senior manager in record time. But after 10 years at the company, she’s been assigned to be the secretary of Iseop Tae, the successor to TK Group. The two entered the company at the same time, but ever since he came second place to her at new employee training, Iseop’s never taken kindly to Mingyeong. She’s better than him in every way and trampled all over his ego… so why can’t he stop thinking about her?