Mmmh, how to answer without spoiling 
Zack developed a crush on Kyara pretty soon after meeting her. He was about 15 at the time, working in the factory. Him and Kyara kinda just started goofing off while on duty and realized they had the same type of gremlin energy for mischief. Combined with her being a cool strong muscle-y lady and yeah, boy fell like a brick. He pretty much tried to find any and all excuses to hang out with her haha.
As for Kyara, girl has some issues haha. She won't realize she has actual feelings for Zack until years later, in fact, she would be 18, going-on 19 before it hits her. And she doesn't even arrive at that conclusion on her own! She needs a certain redheaded girl to make her see sense. It doesn't help that Kyara is also somewhat in denial of her own feelings (reasons for which will be explored in the story). But even then she's really bad at hiding them.
Now as an aside, the Oasis factory accident in which Zack lost his arm also play a big role in the evolution of their relationship dynamic. But the ins and outs of that will be expanded upon within the story itself 
