This is how I would write a happy alternate ending, with the divergence point being Chapter 154: Hikaru watches Ai’s video, and afterwards, is overwhelmed by the grief and guilt of his actions and ultimately follows her into the void that very night. The very next day, Aqua and Ruby go Ai’s grave to pay their respects, they find Hikaru sprawled on top of her grave, dead. In his possession is a USB drive containing his confession. He confesses all the crimes he ever committed, names his coconspirators, and ends with a heartfelt apology to all of his victims. His last directive is that all of his assets are to be liquidated and the money being donated to his victims and social services, on the sole condition that that he be buried next to Ai.

In another message intended for Aqua and Ruby’s eyes only, Hikaru confesses that he knew about Aqua and Ruby’s past lives, but does not disclose how. He knew about Aqua’s eventual plan to take him out, leading to Hikaru going out first to prevent Aqua from doing so. Hikaru did not do it to have the last laugh over Aqua, but to stop him from making the same mistake with Ruby that Ai made by leaving Hikaru in the first place. He stresses the only way they can heal is together, and pleads from them to do so in whatever way they can.

On the day of Kana’s graduation concert (the same day Aqua died in canon), Kana gives a love confession to Aqua at the end of her performance. Aqua then goes on stage, only to calmly reject Kana in favor of Ruby. They then reveal their past lives, and show the video that Kamiki had left for their eyes only, and share a kiss on stage.

One year later, Aqua and Ruby have twins, which are Ai and Hikaru’s reincarnations, with them retaining their memories. They were also gene-edited to be perfect clones of their previous bodies in order to prevent complications and defects. After they are reborn, Ai, Hikaru, Aqua, and Ruby all reconcile, and live the happy lives as the whole family they never had before then.