My Dear Agent, Volume 2 by Ebino Bisque

Professional bodyguard Riichi has recently entered his first relationship ever with Mario Tachibana, the mysterious newbie agent whom Riichi was tasked with training and housing. After discovering Tachibana is actually the illegitimate son of the conglomerate's CEO and his ward's secret brother, Riichi realizes there's a lot he likely doesn't know about his new partner. 

When Tachibana starts acting more aloof and coming home late, Riichi can't help but worry that his boyfriend is hiding more secrets from him. Riichi has trained his entire life to maintain composure and keep a handle on any and all situations that may arise, but in this uncharted new territory, will he be able to protect his most precious first love?