Spoiler (Sorry for replying so late, I didn't see the notice):
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Kiku was granpa's lover. He was a servant of granpa's family. They were in love with each other but granpa was engaged to another girl. Each family must have one person to join the army, granpa's health was not good, so Kiku secretly took his place, without telling everyone in the family. Because of Kiku's disappearance, his grandfather couldn't forget him and couldn't live happily with his wife. That may be the reason why his son - Shirou's father - also couldn't love his wife. You can see in the manga that Shirou doesn't have a happy family.
After getting his memories back, the person Kiku loves after all is still Shirou, but he doesn't want his disappearance to cause suffering for many people. So he chose to go back to the past to break up with granpa, telling granpa to forget about him and love his own family. In the present, the photos show that Kiku now has a happy family, proving that Kiku has changed the past. Shirou went to the lake and threw the paper airplane again, waiting for Kiku to return to his side.
It is DEFINITELY A HAPPY ENDING, though a bit open, but Kiku chose to forget his grandpa and came with Shirou.
Ok after read the raw in Chinese, I can say that it's happy ending. Calm down my friends, it's HE.