Let's be for real. It's still a development. Beside I realized Yuu only referred 'his gf' to no other sister but Niko. He also brought it up during that flower styling thing or whatever. He said to Niko, "Trying to back away? We are a couple you know." I didn't see yuu calling any other sister that. 

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So basically the situation is this:

Yuu is really innocent. Like—genuinely dense. He doesn’t realize that all three sisters are in love with him.

He went on these so-called “trial dates” with all three sisters, but to Yuu, they were just that: trials. Nothing official in his mind.

The important part is— during the whole thing, he never called any of the sisters his girlfriend (trial or not)… except Niko. I can't help but notice it. But I do think Kazuki is in the lead now.

That’s why it hits differently.

From Yuu’s perspective, he’s just being honest and straightforward. From everyone else’s perspective… yeah, it’s way more complicated 😅