Momo
Hinamori

Momo Hinamori is the lieutenant of the 5th Division, and the person Sōsuke Aizen spent decades cultivating as proof of his own decency. She idolised him openly. She wrote to him, defended him to anyone who questioned him, and never noticed that her admiration was one of the instruments he was using.

The betrayal is staged specifically to break her. Aizen fakes his own death, leaves a letter naming Gin Ichimaru as the traitor, and lets Momo find the body impaled on a wall. She attacks Tōshirō Hitsugaya, her closest friend since childhood in the Rukongai, on the strength of forged evidence, and when Aizen reappears alive she runs to him and he runs her through. Hitsugaya's rage at that moment is what gets him beaten too.

She does not recover quickly and the series does not pretend she does. Momo spends the Arrancar arc under medical supervision, refuses for a long time to accept what Aizen is, and has to be stopped from throwing herself at him again. Tobiume, her Zanpakutō, fires bursts of flame from a jagged blade, and her Kidō is genuinely strong. What the character is about, though, is how long it takes to stop loving someone who was never there.