Summary: Secrets aren't so secret when a child can figure them out.
Gin likes watching people. Gin has always liked watching people – he finds other human beings (okay, they weren't really human, but they might as well have been, really) fascinating. And, because he watches, he learns. Gin knows quite a bit about the other members of the Gotei 13, because, while they can keep their mouths shut and their bodies still, their eyes speak volumes. Gin has never liked the idea of people knowing what he's thinking, so he doesn't let them see his eyes. But even when his eyes appear to be closed, he watches.
Gin knows that Lieutenant Kuchiki Sojun is saddened by how distant his son is, and wishes that Byakuya would spend more time with him, rather than his grandfather and the Second Division Captain. He'll never say it, though, because of his pride, and because he doesn't want Byakuya to feel pressured.
Gin knows that Captain Kyoraku Shunsui and his lieutenant, Yadomaru Lisa, aren't really together – they just enjoy flirting with each other. Later, when currently very little Ise Nanao is all grown up, Kyoraku will realize that he doesn't flirt with her to tease her, he really means it. He will, Gin knows, flirt with her even more to cover it up.
Gin knows that Captain Muguruma Kensei doesn't really hate his lieutenant, even though she can be annoying, but he says he does because he isn't quite sure how he feels – he's a tough guy, so he doesn't love her, but he isn't quite sure that it's friendship either. It doesn't matter, though, because he doesn't have to say anything at all – Mashiro already knows.
Gin knows that Hikifune Kirio said goodbye to Hiyori while the girl was asleep as she went off to join the Royal Guard. Gin knows that Captain Unohana's first name wasn't really 'Retsu'. Gin knows that Tōsen has convinced himself that what he wants is justice, because he doesn't want to admit – even to himself – that it's vengeance that he desires.
Yes, Aizen Sōsuke thinks to himself, Gin sure does know a lot. And so, as he watches Gin on the ground, Matsumoto Rangiku sobbing against his chest, pleading for him to live, Aizen wonders if Gin was aware that he would be missed. Rangiku and Kira would miss him, that was obvious to anyone with a brain, but Byakuya and Suì-Fēng, both of whom Gin had spent a large time annoying when they were children, would miss him. Hirako Shinji, who deeply regretted allowing his Third Seat's to fall into Aizen's clutches, would miss him. Sarugaki Hiyori, who had witnessed the effects his pranks had had on both Urahara Kisuke and Kurotsuchi Mayuri, would miss him, although she certainly wouldn't be sorry to see him go, seeing as he cut her in half.
It is a shame, Aizen thinks, that Gin chose to betray him. Part of him had hoped the boy would abandon his vendetta, but such thoughts were for the weak and naïve. And, well, Aizen has better things to do than reminisce on impossible possibilities – such as showing Kurosaki Ichigo just how outclassed he really is.
Lying on the ground, bleeding, Gin smirks to himself. Ichigo will win, he knows, because, like himself with Rangiku, Ichigo has someone to fight for. Ichigo has a lot of people to fight for. Aizen was very observant, but Gin was the one who was always watching. Don't die, Ichigo, he thinks. Don't be like me. Don't fail that poor girl like I did. And, miraculously, Ichigo looks over at him and gives him a nod.
I'll win, he promises silently. I'll win for you, for Rangiku-san, for Inoue, and Ishida, and Rukia, and Renji, and everybody else this bastard hurt. I'll win. His eyes have gotten stronger, Gin notes. Yeah.
Kurosaki Ichigo… the rest… is up to you…