Author Chapter 157 (KAKAO 79%)
The probability of him being Ai's half brother is like 80% as his story about his parents fits. (i have another theory that maybe it's Ai's ex-boyfriend from the last school but it's kinda weird to keep in touch with an ex so.... who knows)
And about Ai, i do think that the way she was bullied was sad but that does not make Yuu and Tsubasa completely at fault for what happened to her. Yuu's backlash at her was to be expected and honestly i don't think his words were wrong (though it did hurt her but as Yuu said, she chose her feeling over Tsubasa unlike Tsubasa so this is her fault, especially in that situation).
So in all of this Mr. Orange head does not have any right to get in their way or extract revenge from them. They both lost something equally important along with Ai.
They should've just ignored this guy but because of the way they are, they just can't. Especially Tsubasa, she's too pure ? It's probably a name change; Natsume is his step-dad's last name.