On page 8, Chi's LINE avatar is a picture of an ant. All the way back in chapter 74 (page 3) Yamada was telling Ichikawa some story about Chi doing something with ants.
Always has been. The novella epilogue makes it absolutely clear she's been strongly drawn to him from the very start...and that "the very start" was way before we all thought it was.
You can square the two by saying that Yamada was attracted to Ichikawa, but wasn't yet aware of this attraction until chapter 27, when Ichikawa buys her the Milk Tea during the workplace visit. It's a bit more obvious from Ichikawa's point of view where "I want to kill her" is obviously just his chūnibyō tendencies layered over his real feelings.
Ichikawa's family is insanely rich. His older sis goes to Waseda which is like a really high ranking private uni that's really expensive, their house is a landed one, and he's planning to go to a private HS... Damn!