Miyuki Ayukawa is Dan's childhood sweetheart who moved away from his neighborhood when they were children. She returned in the first episode as a well endowed teenager and a skilled mechanic. Upon her return she gives him and teaches him how to pilot a BigFoot.
She cares for Dan and his team's Big Foots, something she does happily, as it gives her more work to do (and get paid for). She also has been shown as having deep feelings for Dan even though he doesn't notice them. Later, she redesigns the team's BigFoots, making them move much faster and smoother than before, enough to create an afterimage.
Though she is not a basquasher, she is still an irreplaceable part of the team, constantly fixing and upgrading all of their BigFoots. Though she has feelings for Dan, she steps aside once Rouge comes into the picture, and accepts their mutual attraction.
She did not participate in the Legend Tournament, but she did go with everyone, aand did the finishing touches on the BigFoots. Her upgrades were overwhelming; the shoes that Haruka made for Dan were burned off after he pushed the BigFoot to its limits. In the aftermath, she is seen working with her grandfather on an orange BigFoot (it is highly probably that it is Dan's), showing that she will continue to be a mechanic.