The progression of realization
Page 14: The reader's first glimpse 

Page 17: Zen notices (hehehe... his precious blushing 

)
Page 18: Oh my! 

Page 19: Yae is a dense, dense block of ultra firm tofu 

Page 29: Yae finally notices! 
 
After the chapter: Are we the readers like... Zen's late mother, watching over our son and his childhood friend as their relationship slowly blooms? It makes me (almost) consider having kids...