Rin isn't sure when it started. Was it the other day, when he spotted Suguro sitting on a wall—taking a breather from being Lightning's maid undoubtedly—silhouetted in the sunlight? (When he sees him, surrounded by light as he sits and meditates, Rin's breath catches in his throat, though he isn't sure why.)
Was it maybe when Suguro showed up to class one day, his streak of blonde hair completely gone, and the back of his neck shaved? (Rin remembers Shima throwing a fit, and Konekomaru forgoing a comment to roll his eyes at Shima. But mostly he remembers seeing that haircut and Suguro's embarrassed and irritated expression, and having to stop himself from blurting out a barrage of compliments. His excitement wins out though, in the end, and Rin isn't sure what the tightening in his navel means when Suguro's cheeks pink at the eager praise.)
It could've been when Rin was witness to Suguro's desperate attempt to take Lightning as a master, watching in shock as he prostrates himself before the man. (Rin isn't sure he can name all the emotions that were surging through him, seeing Suguro so passionate and upset and pleading. He hears Suguro instantly dismiss any notion of taking Kurikara from him, even though it would mean gaining Lightning's tutorship, and feels his heart lurch uncomfortably in his chest. He watches in awe from inside the cursed box as Suguro performs the task Lightning drops on him; gapes unseen as Suguro demonstrates the masterful grasp he has of all the things they've learned in class—and then some. The pride swelling in Rin's chest feels explosive.)
Perhaps it was one of the days that Rin ate lunch with the Kyoto trio—Suguro had forgone getting himself food because of something stupid like studying, so Rin plops down next to him and shares his homemade bento with him. (Sitting there, nearly thigh to thigh with Suguro, Rin remembers his throat feeling inexplicably tight. He gives up on sharing and shoves the entire bento into Suguro's startled hands. He tells himself it's him just being extraordinarily pleased with himself that his face heats up when Suguro exclaims aloud at how good the food is.)
Or maybe it was way back during their first trip to Kyoto, when Rin and Suguro go off to defeat the Impure King together, and Suguro looks up at him and quite literally places his life in Rin's hands. (Rin doesn't think it's likely he'll ever forget that moment, those words: "we're friends after all. I'll trust you", the look on Suguro's face as he says them. He thinks the whole world might've stopped for a second in that moment, as they stared at each other.)
Rin also thinks it could've been from the very beginning. Back when Suguro still had an incredibly aggressive vendetta against him. (He remembers hearing another boy in class, muttering angrily under his breath; remembers turning and peeking over his shoulder to see just who was talking about him now. The hair, the piercings, the fierce glare pinned directly on him—Rin's mouth hangs open just a smidge and something in his sleepy brain goes oh. He's entirely unaware of it.)
In the end, Rin supposes it doesn't really matter where it started. As long as it doesn't end, that's fine by him. After all, he'd realized it eventually. (Ryuuji still likes to tease him about being slow on the uptake. "I think we were dating for a few weeks before you managed to figure it out," he'd say with a snarky grin. "Did you think I liked to make out with all my close friends after class?" Rin always scowls at him, flushing in embarrassment, and kisses him. That'll shut him up, at least for a little while.)