
I can't understand chinese so take this with a grain of salt
SPOILERS
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From what I heard the ending is bittersweet.
Kiku is gramps lover who he believed "died" when his plane "crashed" in the ocean hence why he wanted his ashes to be scattered at the sea. Jirou was not a good husband and father to his family because he couldn't move on from Kiku's death. His actions might have been the reason why Shirou's parents died? I think I saw someone say his parents committed lovers suicide or something and Jirou tries to atone by raising Shirou with love. I guess his sin might have been his inability to love his family which is probably why the uncle had that look on his face when Kiku asked if gramps was a good father to him and his sister when they were talking about Shirou's childhood.
Anyway I don't know if the uncle convinced Kiku or if it was Kiku's decision but it seems like Kiku went back so he could settle things with Jirou cause he feels guilty and wishes Shirou didn't lose his parents. Apparently Jirou was suppose to be engage? but instead of talking it out so they could have a proper closure Kiku runs away to the military. Shirou was not having this fix the past nonsense but somehow Kiku managed to convinced him. To send Kiku back Shirou has to do the same thing he did to summon him: throw that old plane at the lake. At the day of Kiku's departure I think Kiku confess his love and promised to come back since he want to stay with Shirou? He disappears after Shirou hugs him and we see gramps + Shirou photograph was replaced by Shirou + his parents which means Kiku succeeded. Shirou says Kiku got his wish so now it's his turn to get his wish to come true. He throws the plane to summon Kiku back. Since this worked twice already it's assumed they reunited even though we didn't get to see it ╥﹏╥
The limited edition in vol 1 and 2 has a bonus story. The link contains vol 1 bonus about Shirou's newly awakened desire for Kiku. Vol 2 bonus first half they were talking about their birthday but since Kiku doesn't have his memories yet Shirou says they can celebrate it everyday or something. Second half after Kiku's memories returned he tells Shirou his birthday. They say they'll celebrate it soon and that they wish to be together forever.
Does anyone have the link of raw? Please