Create Nicknames for Friends (Your Eyes Only)

Some of my friends, thankfully, have their usernames set as their real name. Others will use their gamer name, their pseudonym, or something else entirely and I'll have trouble remembering who I'm speaking with (if we're not the best of friends). For now, I've solved this issue by writing the person's real name in the "Profile Notes" section of each friend. But if I'm looking to direct message a friend I haven't chatted with in forever and forget that friend's specific username... it takes forever, sometimes never finding my friend. 

Could you provide a function where we, the user, could give our friends nicknames that only we see? It could even be as simple as hovering your cursor over their username to reveal your custom nickname for them.