My man Ryuki is insanely fucked up but the core of his problems, the one thing from which all his other issues stem from, lays in the fact that he has absolutely no sense of self. He got his ideal from his brother. He got his car from his parents. Even his dedication to the HB case comes across less as "I want to do a good job" and more as "I can't disappoint Mr Date he has high expectations for me." Ryuki is a man who carries all his ghosts with him to the point where there is little room for himself as a person. If you remember my post comparing AINI and AITSF, Ryuki is the embodiment of "holding on;" craddling his dead loved ones inside his ribcage until there's nothing left of him.
Meanwhile, Date has the opposite problem in that he has too much of a sense of self. "Kaname Date" as a person isn't supposed to exist. He's a fake! Boss made him up when she had to give her friend a new identity!! But Date has grown into that skin now. He is no more Falco than he is Rohan or Saito. "Kaname Date" is a liminal person existing at the cross-section of all these people. He's fourty, thirty, and six years old all at once. He's a serial killer twice over and he has never taken a life with his own hands. It would be so much easier if he could just go back to who Falco used to be- but this is impossible. He is no longer that person. This is why he refuses that name at the end of AITSF. This is why he wears a face mask in AINI. He has no body of his own and three people worth of memories, nothing about him is even supposed to exist, but unfortunately, here he is. If Ryuki is "holding on," Date is "let go;" he's the man who moved on with his life and grew as Kaname Date, for better and for worse.