She's so self sacrificing, even after she finds out the truth. She still goes and sacrifices her happiness for everyone else.
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This was more annoying than satisfying to read. Her dad felt if they didn't do this, he could lose his job, but would rather sacrifice his daughter who already sacrificed herself to take care of them. And I get it, culturally that is a thing that happens, but still that is your child. Then there's Marino who couldn't meet up with the guy just to turn him down because she was already seeing someone and couldn't be bothered, but she seems like the personality type who can reject people. So they leave it all up to Yukino to deal with, disregarding any feeling she has, including how uncomfortable she is with the situation. (╯°Д °)╯╧╧
Then there's al, who knew. He probably knew from the very beginning who Hikari fell in love with as soon as he told him and he investigated it. Instead we get this story of Hikari pushing his feelings, and yukino hiding hers all for the sake of everyone else.
Sekaiichi Suteki na Propose Kimi wa Ore no Subete