This is my second fanfiction so it probably really sucks, I'm sorry beforehand. This is from Shinji's point of view.
Disclaimer: I do not own the characters in this story, they belong to Tite Kubo
Shinji's found that he's actually quite an observant person, especially when it comes to the other visoreds, and that explains why he's the only one out of all of them that notices Kensei's total interest in the vice captain of the 9th division.
Kensei once told them all that he saved some kid from a hollow the day they were hollowfied and Shinji knows he always wondered what happened to that kid. So, when the war is over and he looks around to see how everyone is doing and he locks his eyes on Kensei talking to what seems to be some random guy, he's only confused for an instant. The second the guy turns his face to the side a little and Shinji sees the tattoo on his face, he puts it all together.
It's no surprise to him when Kensei tells them all that the guy is the vice captain of the 9th division. It's so obvious that he became a shinigami because of Kensei, it's no wonder that the guy joined the same division that Kensei had been in.
Later when they're asked to resume their old positions in the soul society and become captains of the Gotei 13 again and Kensei practically jumps at the chance, Shinji has to turn his face to hide the smirk creeping onto it.
What surprises him the most about Kensei being interested in Shuuhei Hisagi is the fact that he doesn't tell any of the other visoreds about it. Kensei doesn't usually actually tell them if he likes someone, but he also doesn't hide it from them either. When Shinji heads over to the 9th division to ask him about it and sees him talking Shuuhei, the way Kensei's looking at the guy clues him in to why Kensei hasn't said anything about it. It's obvious to Shinji that Kensei hasn't told anyone because he's completely serious about the guy.
He's seen the two of them together, seen how the feelings are so obviously mutual and he knows that Shuuhei and Kensei have an age gap between them, but Shinji honestly thinks they're meant for each other and that's all the incentive he needs. So, when they're out drinking with Rose, he waits until Rose goes to the bathroom to look Kensei straight in the eyes and tell him he knows about it and before he can deny anything, Shinji gives him his full support and tells him to go for it because he knows that's all Kensei will need to make a move.
After that, he waits for Kensei to tell him any updates and when he finds a piece of paper on his desk that just says 'Thanks', Shinji smiles.