Edited in 30/09/2023
Figure
The fact that Nanjiroh could never replace Kiritsugu as the person that he saw as his father was something that Ryoma had come to accept the moment he recognized that he used to be Shirou Emiya
For him, Nanjiroh was more like an older "friend" who helped him to recognize that tennis was something he could enjoy on top of the other interests he might have. Never did Ryoma ever recognize Nanjiroh's ideals or dreams as possibly his own, and neither did he feel like he owed Nanjiroh his entire existence.
(In many ways, it was Kiritsugu that gave him his life as Shirou Emiya. It was he who pulled him out of the burning wreckage that erased everything he might have before the fire. Kiritsugu was the man he could never come to resent despite his ideals being the one who brought him his demise.)
He never expected Nanjiroh to be one, after all. One was a retired pro tennis player, the other was a magus killer whose life might rival his previous life in terms of tragedies and pain.
(Emiyas was born to suffer, Rin joked once when they unraveled the mysteries of Kiritsugu's life. The despair in Shirou's eyes caused her to shut herself up.)
It's such a shame that Rinko was barely around most of the time. Having a mother figure around would have been nice for once, though Ryoma never found himself wanting something he never had and would probably barely have for the rest of his life.
(Once upon a time, Illya told him of a mother that she had. The mother who loved her, the mother who was there with Kiritsugu for her. Shirou, so tired from the Holy Grail War at the time, told her that she was an idiot for being jealous of him for having Kiritsugu for a bit of time when she had both Kiritsugu and Iris for longer. Illya agreed)
Connection
Ryuuzaki Sakuno was one anomaly that he found in his new life. Ryoma knew better than to look for ghosts, to look for connection where there wasn't and would never be.
But looking at the admiration that Sakuno seemed to give to him almost unconditionally, Ryoma couldn't help but push her away as the image of a purple-haired girl who called him 'Senpai' always seemed to overlap with hers and he couldn't find it in himself to do that to them, to both Sakuno and Sakura.
(It was his failure that caused Sakura's death. It was his own two hands that took her life. Yet she still found it in herself, in the middle of all the pain and insanity, to thank him for that.)
It was for the better, anyway. Ryoma knew that it would never last, no matter how many times Nanjiroh and Ryoga told him that they deserved each other, or how many times Sakuno's grandmother wanted him to be the one for her. He would always say no, because he would never stop seeing Sakura within her and he knew that subconsciously, Sakuno knew that as well.
So when Ryoma heard that Sakuno had found someone, he couldn't help but feel relieved.
(In many ways, he's still very much afraid that being involved with him will endanger someone. It's silly, as Ryoma knew that unlike Shirou Emiya he was not gonna be prancing around the globe hunting down terrorists or picking fights with warlords, but the paranoia remained nonetheless)
Parallel
Ryoma met Rin Tohsaka of his world completely by accident. Except she's not a Tohsaka or even a Rin. In fact, Ryoma had no idea what her name was. The only reason he didn't think that she was his (or more exactly Shirou Emiya's) was because he remembered that Kaleidoscope could only access parallel worlds and not parallel universes. This was to say that since the way that "magic" works in his new world was very different from the one of his last one, they didn't share the same universe.
Strangely enough, it was in a McDonalds that he met her. In all honesty, Ryoma hated fast food. Even as Shirou Emiya pranced around the globe hopping from one warzone to another, he always made it a priority to make his own food and only go for fast food when absolutely necessary. Not to mention he could easily make a much healthier and tastier version of things like burgers, fried chicken, and fries himself. But neither Ryoga nor Momoshiro nor Eiji listened to him when he said that.
He remembered that Rin hated fast food, claiming that it was not befitting for a lady like him. So he was surprised when he saw the girl who looked exactly like the Rin Tohsaka he knew come into the joint. She ordered some burgers and fries to takeout before leaving as fast as she came.
His older brother and seniors teased him later for staring at a pretty girl for so long, but Ryoma simply shrugged and said that she reminded him of someone he knew.
When he returned home, Ryoma couldn't help but wonder if there was also a Shirou Emiya doppelganger in his world.
(His answer would come when he was already 25 years old, staring at a ten years old, red-haired boy who looked exactly like he was in his previous life before the young boy was pulled away by his mother)
Trace
Ryoma realized that while he was no longer able to figure out everything about a sword simply by looking at it, he could still do the same by touching it. He realized it when Genichiro allowed him to touch his family's heritage sword during his visit to the older boy's house when he was ten years old. He immersed himself in the sword for some time, long enough to make his fellow kendoka worried about him and shake him awake.
(It was then that Ryoma duly noted that within him still lies Unlimited Blade Works, just out of his reach due to the fact that his world rejected the idea of projecting such a world or anything within it into the "real" world)
Since then, Ryoma found himself looking around museums and dojos for swords for him to touch and immerse himself in. It was something that he missed as if his very own soul was calling out to him to look for swords, to connect with one. Nanjiroh joked that one day he might end up being an actual samurai instead of just having a nickname as one like him. Ryoma told him that he was an idiot.
(Ryoma knew for one that samurai weren't all that noble if the history he traced from their swords was any indication.)
At the age of 12, Ryoma found himself named as one of the youngest sword appraisers in Japan, much to his own ire.
(He knew that blabbing out about swords to Genichiro's family was a terrible idea. After all, it was them who became his guarantor of sorts for his appraisals, the ones that make others only blink once and take it in stride when a pre-pubescent teen comes in and was the one doing those appraisals)
Understanding
It was not long after Ryoma defeated him that Nanjiroh came to him and asked straight to his face if he was really his son.
"By all means, I am Ryoma Echizen and have always been." He replied back then and Nanjiroh never asked again, letting him do whatever he wanted while vaguely acting like a father to him at times. For the most part, however, out of sight from anyone else, Nanjiroh treated him as his equal, never really treating him like a child since then.
Yumiko Fuji might be the first person that Ryoma ever told about his past life, but deep down, he knew that Nanjiroh didn't treat him that way just because he defeated him in tennis. Ryoma knew that in some ways, Nanjiroh understood that while he was Ryoma Echizen, he was no longer his son. At least, not the one he knew before something within him suddenly changed.
Ryoma never saw Nanjiroh as his father, and Nanjiroh understood to some extent that Ryoma wasn't his son. And for that, Ryoma was glad. Until he met Yumiko and Syusuke, there was nowhere else where he could loosen up on being a child that he knew he wasn't, he never was.
(The fire was the end of his childhood. Whatever he has after that was only an illusion kept up by Kiritsugu and Taiga's misguided perception of the Emiya family)
Nanjiroh never was and will never be his father, but Ryoma might find it in himself to someday consider him a friend.
