Everyone has their interpretation of Taro and Sonoi. The series blog said that Sonoi has feelings for Taro, they named the official ship (TaroSono) and even said "You love him, don't you?". It's up to you to decide whether or not you like the ship.
I think Rei's secret was that he tried to kill Hajime as a child, because Rina liked him, and out of jealousy he tried to get rid of his friend. I deduced this based on the jealousy scenes and the scene where he cries for never having seen Rina cry. I don't know if it makes sense, but that's what I was able to understand.
I also believe it to be canon. They're cute together (Sonoi Actor's Own Words).
Tags are gone :(((.
I don't know if it makes sense, but that's what I was able to understand.