hikaru is the focal character of the series, so you'd think she'd be the first to be hooked up with a love interest. if not her, then it would have to be umi, who is confident, graceful and beautiful. yet it's the soft-spoken nerd who winds up falling head-over-heels, and with someone who initially appears to be anything but her type!
the girls meet ferio in episode three. he works his charms on fuu, hoping to use her feelings to keep tabs on the magic knights. after finding himself about to give his life for her in episode seven, he realizes that he really does care for her, and their love begins.
the knights are required to mature their hearts as they journey to awaken the legendary mashin. each great test of their hearts matures them bit by bit. fuu is the first to evolve her sword through her will and her emotional and mental growth. her desire to save ferio's life was strong enough to trigger the evolution of her weapon. obviously, her sword would not have evolved so soon without the inspiration of her feelings for him.
ferio genuinely wants the knights to save cephiro, but the reason he desires to keep meeting up with them comes back to fuu. he has an obvious affection for her and wants to check up on her. in episode eleven, he puts his safety on the line yet again to help the knights by distracting caldina so the girls could escape. when fuu asks him why he'd do this for them, he replies that it's because he'd fallen in love with her.
in episode fifteen, probably the most volatile between the two, fuu wrestles with unfounded jealousy and the weight of choosing between ferio and the good of cephiro. being a magic knight, fuu is strong enough to know that although she loves ferio, cephiro is more important, and that he would be disappointed if she'd sacrificed cephiro for him in the end. she's not his sister, after all.
while it can be argued that the story relies too much on fuu's growth being centered around her love for ferio, fuu demonstrates her strength of heart for her friends and for cephiro on many occasions. but yes, fuu does center much of her growth of heart around ferio. he is a very significant part of her heart and it only makes sense that so much of her maturation relates to her love for him. much of her growth as a knight is attributed to what she and ferio have together. and hikaru and umi need fuu. the three girls make a perfectly balanced team.