In "The Ultimate Mercenary," the male protagonist, Ye Hao, returned to the country to protect his former sister-in-law, Li Xin, after retiring from the army due to a strange illness. He also contracted a marriage contract with the beautiful president, Lin Jiatong, and became the king of mercenaries. This book matched the plot where the male protagonist was a mercenary and his girlfriend had a relationship with her teammates. I hope you like my recommendation. Muah ~