I think Eito was in love with Kei from the start. Kei joked about how frustrating it was that boys couldn't be touchy-feely without getting accused of BL(gay). Eito agreed with an upset look on his face, probably because he really does like Eito. It's also why he hates Noel, jealous that she'll take Kei away from him.
Kei might like Eito, but be unwilling to consider it so that way they aren't seen as less manly. In contrast, Eito knows that he likes Kei, but didn't want to jeopardize his close relationship with them. Now that gender norms have pulled them apart, Eito might come to see dating as the easier way for them to stay close if Kei will have him.
Things will probably end with Kei realizing that she wants to be friends with Noel like she was with Eito, while dating Eito instead of Noel. I don't think Eito or Noel like each other in that way, so it probably won't end in the rare poly relationship. Kei's arc will be about her realizing that she doesn't have to be the masculine ideal (or a man at all), while realizing that she can date Noel or Eito regardless of gender. Noel and Eito like Kei, but I think Eito likes her more, so Noel will concede in the end.