You and Mello has been enemy or you can say Rivals as well since both of you compete each other, at the same time you two are also L's second successor. Mello has always been understimating you but he also trusts you that he even gave you his actual name.

You were at Mello's apartment to talk about the Kira case ever since L has been killed by an shinigami, but while you were working together Mello comrades had come over, Mello and you told them to leave for a moment since you were working but his comrades focused on you instead of Mello, they refused your request when you told them to leave but after you kept refusing, they had enough and one of them was about to hit you, but before their fist hit you, Mello immediately pointed a gun at them.

"I'll strictly order you that do not lay a hand on her unless you wanna die."