There's not really any abnormality there. Rea didn't interact with any guys her age, so anything Chihiro did would have intrigued her, all the more so because he was so weird and thus so novel. Rea is the first real (humanoid) zombie for Chihiro, so naturally he's now interested only in her. Rea, however, got over the initial "wow, I'm talking with a boy" phase and now tries to do everything she couldn't do before.
If there's one mistake here, it's Chihiro being so concentrated on Rea. He can't very well carelessly experiment with her, so he should either tamper with Babu more or preferably make a larger batch of zombie animals for pure experimentation. Occasionally he displays behavior fitting for a necromancer, like his disinterest in spending an hour waiting for Rea to shop as opposed to making his observations. But then he reverts quickly to a normal dude by blushing and getting embarrassed in Rea's company. I'd like him to stay in the role of a cool scientist more. I'm sure it wouldn't fulfill any shounen romanticist's expectations, but it would also allow a rather cold romance conclusion of Rea accepting her role as a necromancer's fetish and experiment, but little more, whereas Wanko would get a human relationship with Chihiro, though lacking any passion returned.
Rea's old man practicing European fencing instead of Japanese sword arts does indeed make him look even crazier in his dignified and orderly manner. More detached from normality.