Not Quite Married: Doted on by My High Class Childhood Friend

Tsumugi is a standard employee at a global finance firm, and her childhood friend, Ritsu, sits as the member of the board. To her, he has always been the reliable "brother" figure-until the day he drops to one knee.
Tsumugi is stunned when Ritsu pulls her close. "I'm going to prove to you that I'm not your brother," he whispers. As he kisses her, the calm mask shatters, revealing a gaze as hard and brilliant as a diamond. For the first time, Tsumugi doesn't see a friend; she sees a man who will stop at nothing to claim what's his.

(Source: MangaPlaza)