Disclaimer: I so don't own it.
All three of them enlisted in the fifth squad after completing their training at the academy. Aizen was quite pleased at having spotted them. He would have to remember to thank someone for his good fortune later, as all three showed great promise. He had taken care to cultivate them after that awful night, kept in touch with them all right up to graduation. Impromptu visits, offers to come and shadow with his seated subordinates, flowing and unstinting encouragement and praise. All these things guaranteed him their rapt loyalty. And he basked in it. Within weeks, however, he knew he had to reconsider his decision. They were all young and pliable, for the most part, but little idiosyncrasies they seemed to exhibit made him wary.
Izuru-kun was exceptional at all he did. However, he seemed high-strung and any criticism—constructive or otherwise—seemed to sap his self-confidence. He was clearly haunted and shamed by his fear of death, which could be a fatal flaw to a shinigami. Aizen was careful when he directed the boy, always a smile and a warm hand on the shoulder, but Izuru-kun remained vulnerable to the opinions of his peers. Particularly the redhead.
Momo was overflowing with self-confidence. She performed her duties ably and well, was courteous and well-liked by all of her peers, and never faltered in her devotion to the squad and her captain. Nevertheless, she also showed signs of a budding attraction, which shocked all those around her. She was constantly in his company, and often her conversations were peppered with comments about what this man said, thought, did. It was no secret she was attracted to power, and not in an impure way. Aizen understood that gaining and maintaining power meant surrounding yourself with powerful people. Which Momo did, perhaps even unaware of it herself. But of all the powerful people she was exposed to, she was drawn most to him. That redhead.
Abarai Renji came to the fifth with a reputation. Loud, brash, almost arrogant. However, he backed that up with an almost zealous attention to his duties and rarely failed at anything. On the rare occasions when he did slip up, he was his own harshest critic. But again, those occasions were rare. He impressed even the seated officers above him, and it was clear who, if the three were left to their own devices, would be rising in the ranks the fastest. Which would put him in a position above the two Aizen had chosen so carefully along with him. And Aizen knew well enough he would ride them, hard, not out of a tendency to bully (although Aizen did not doubt the redhead possessed one) but out of an earnest desire to help them be their best. But the best Abarai Renji desired for himself and his friends differed in fundamental ways from the best Aizen wanted for them.
They did come as a set, Aizen mused. It was how he found them, puppies growling in a corner, facing down a danger they surely were not strong enough to overcome themselves. The bond that forms between people who face life-and-death situations together is natural and healthy. However, the continuation and likely strengthening of that bond was clearly becoming something he could not ignore. Or even allow to happen.
Abarai Renji had to go.