Kasumi no Hana wa Ao ni Hodokeru

After finishing her life as a human, Saika is reborn as a fairy in the world of immortals. However, she is unable to adapt to the world of immortals, so when she descends to the human world, a man suddenly appears. This man, Kouzen, whose life was saved by Saika when he was a boy, declares, "I will never leave your side for the rest of my life"!

(Source: Shogakukan, translated)