Hideki's reaction when Kojima said nonchalant things about how he envied Hideki for having such a high-performance persocom but didn't use her to full potential. "WILL YOU STOP TALKING ABOUT HER LIKE SHE'S A MACHINE!! PERSOCOM THIS, PERSOCOM THAT. CHI HAS A NAME!! DON'T LUMP HER WITH THE REST AT YOUR LEISURE!!!"
And then his realization at what he just said, and the fact that he comes to realize he meant it just as he said it.
Yumi's realization that Ueda loves her for what she is.
"It's not that I love her because I want to make love with her, but I want to make love to her because I love her."
Every time Hideki cries out Chi's name when she is in danger. Combined with the fact that the epic Dark Chi Theme is usually playing in the background you can truly feel how desperately he fears losing her. And the fact that in those tense moments with shady characters lurking, this generally spineless coward fears nothing else.
Chi learned from Mr Ueda that it's not appropriate to hug just anybody, that it should be saved for someone you care about. Later that day, in a personal moment, Chi asks Hideki if it's ok to hug him. Hideki is puzzled since she never asked him this before, but says ok. Chi then hugs him tight as the episode ends.
Chi's "Most precious thing" speech to Hideki when he gets her from Dragonfly's lair.