Natsuka thought he was Haku but the bonding time he had with the 'person' was with Seno. ... his personality, his words, his thoughts.
Natsuka doesn't know Haku but superficially since they were never close so even when he finds something 'Haku' is saying or doing is not what he expected, he has no way to know it so you can see it this way ... Natsuka has spend this time idolizing Haky from afar and having an idea of what it is to be with him and this unrequited love never allowed him to even given any other kind of attention to Seno. Now that he thinks he is with Haku, he is allowing this person to get close and he is giving attention and spending time with him thinking that he is getting 'to know' the real Haku.
You can see it this way... Natsuka is falling in love with this person that is getting to know now ... this person who laughs at the way he handled his family, this person that he is engaging in conversation, his soul ... and it turns out that it's Seno ( because Seno can only say he is Haku but he can't imitate Haku after all since he also isn't that close). It's Seno's personality and traits because Haku is not really this person.
I don't understand how Natsuki fall in love with seno if Natsuki thought he was haku the whole time I just don't get it????? I was rooting for Natsuki and haku the whole time. I read the raw I didn't understand it but I definitely knew what was going on, but I was convinced that Haku was in seno body for those 2 months. I don't know I guess I have to wait for the rest to be translated but I already know that I'm not going to be satisfied with the couple at the end （￣へ￣）
(I guess I should've known who was going to end up Natsuki in the end seno face is on the cover of the manga after all)(￣∇￣")