Ritsuka is surely not a replacement. I'm more thankful that you can't relate with Mafuyu. It may be different for you or sumtn, but I gotta say as I'm grieving i see glimpses of my dead loved one(s) very often, it took me around 5 years to finally had it a bit easier. Meanwhile, GIVEN started around 1 or 2 years after Yuki's death i think. I believe it's normal for Mafuyu to still see Yuki around in places or other people.
To begin with, losing a boyfriend from a normal breakup where you can at least have the idea that they're still living well somewhere out there is very different from losing him bc he died. Especially after such a fight, more over as unexpectedly as Yuki's death. The emotions are pouring over for someone who's buried under cold dirt.
Mafuyu was already that very different Mafuyu when he met Ritsuka. He was already a Mafuyu that had lost an big part of himself, and he's continuing living not only for himself by then. Ritsuka fell for that version of Mafuyu, he knew, he acknowledged Mafuyu's grief, and he accepted everything. Bc Ritsuka is that kind of person, very kind, straightforward, and unwavering. It's useless even trying to rival a dead man after all, it'd only hurt Mafuyu too. The love Mafuyu has for Ritsuka is completely different from the one for Yuki, it's not fair to call it a replacement
This is too long, sorry for my yapping! if you actually read this far, thank you and i hope for the best in life for you!
Because i could not get it off my mind that "i feel" like ritsuka is a replacement. The scenes where he sees a glimpse or often remember yuki when he looks at ritsuka. I dont like the fact the he plays for yukis old band. Im happy that mafuyu is healing but i think ritsuka deserves better treatment than that. It felt like he was always under Yuki's shadow. I feel really bad and commend ritsuka for being patient.
Well right now i dont know what part the story currently is.