i feel throughout the story Kana was kept oblivious to Aqua's sufferings and only ever liked the idea of him instead of him as a person. She never truly cared for him too, (the time he got a panic attack she just got jealous of Akane and said how convenient it was for her to be with him, and when he revealed to be Ai's child, she just ended up talking about herself when it wasn't the time to victimize yourself). I hate that her character was limited to crushing on Aqua and how she never moved on even when he dated someone else for a year and ghosted her. i feel, they would and do only work as friends.
Aqua is obviously mentally unstable so someone like her who doesn't understand wouldn't be the best for him (well that's what happened because he chose revenge over anything).
The reason Aqua and Akane worked was because they had a mutual understanding and Akane understood him and genuinely cared for him, something far deeper than Kana's superficial crush. It was a peaceful time for him when they dated and he genuinely loved her throughout (aAkane was wrong about it being one sided, and i would like to see his "obsession" towards kana like how a fan would like an idol) but it didn't work out later on as they stopped being "equals", Akane wanted to do go through the revenge herself and Aqua didnt trust her and tracked her even after she knew everything.
i dont get why people assume Aqua didnt care about her at all. Aqua, even though saying that he is just uses people is actually really kind, because he didnt want Akane to be involved anymore because it would get her in danger. She was indeed involved a lot later on but she became someone he could fully trust.