It's difficult to know Zhan Zheng Xi's feelings because he's stoic (unkike Jian Yi who is expressive), Plus, remember feelings sometimes are confusing, maybe XiXi doesn't even understand the extent of his own emotions towards his best friend... But I think the author is implying they will develope mutual romantic feelings, because you are forgeting that Zhan ZHeng Xi has blushed furiously with Jian Yi in various occasions... like when Jian Yi tryied to find a nickname and called him "XI", when they picked up somethung from the floor and accidentaly touched hands, and various other times, also XiXi showed him a night sky view from a hill as a birthday present (this is pretty romantic, it's not the type of present you give to a friend....but he doesn't seem to notice himself these things), and he gives in Jian Yi's whims when he pouts...
I agree with the user above but I also always thought the love that Zheng Xi has for Jian Yi wasn't just friendship-love, he just kinda didn't give it much of a deeper thought? Like, when Yi stayed over and Xi said he thought Yi was "good-looking" or when Yi rolled over him and they sorta hugged for a little there and Xi was all flustered and confused afterwards.
If he really didn't love him, I think the aftermath of Yi's confession would've shown us even just a little less intimacy coming from Xi's side, but we only see him showing more affection than anything else (which do not translate to "I'm sorry but no" to me, really, lol) so it could probably be meant to lead to something else..
I just feel like Zheng Xi just want to protect Jian Yi. I dont feel like he's sexually attracted to Jian Yi. Jian Yi heart skips a beat when he sees Zheng Xi but what about Zheng xi ??? How does Zheng Xi really feel?!? (/TДT)/ But at the same time Jian Yi only kissed him the night before. Maybe he just needs time? ┑( ̄Д  ̄)┍ Sometimes I forget Zheng Xi only found out Jian Yi's true feeling barely 24 hour ago. It just feels like forever cause the update times are so spaced out but I need a sign from ZX! Anything! A blush? A Heartbeat? A second kiss! Something lol (▰˘◡˘▰)