You could combo Explode with Celestiary if Nozomu's all-rounding. Personally, I never used it.
Earlier, Ehrjegel posted some stuff about the affection points system. Unfortunately, it's not exactly accurate.
I've reached Chapter 8. According to that earlier post, there was supposed to be a trip into town with the girl with the highest affection aside from Satsuki and Nozomi.
Using the math, I was expecting to go with Ruptuna but I ended with Naya.
I wonder if there are some other parameters which I don't know about.