I would honestly understand "if and only if" the author introduces Shinoa as another childhood friend of Yuu
Then this MikaYuu that we're shipping will turn to brotherhood while Shinoa could've been the only family left to Yuu when he thought he lost everyone while he only have Shinoa left as family, he would be so protective of her.
We can see that Yuu hold grudges because he loves his family so much that he's willing to go against everyone and make them his enemy if it means to bring his family back
So I came back here to read one more time but I just couldn't make sense how Shinoa turned to a generic 360° simping btch.
You can say that Yuu and Mika has more couple chemistry than Yuu and Shinoa which is literally NONE EXISTENT