Imagine Kaede being mute after damaging her vocal cords from some accident as a child and have trouble trying to trust after being bullied as a kid, because her disability is a burden, thus ends up being homeschooled during her middle school years. She gets accepted to Hope's Peak as the Ultimate Pianist. Kiibo being installed with sign language helps her throughout her day. As a side lesson everyone took sign language to communicate with Kaede.
Aw, poor baby! Yet, I think she would still be outgoing and friendly, or at least try to remain cheerful and optimistic, even if she is a little cautious around the others because she’s mute. But a friendship between mute!Kaede and Kiibo sounds like a beautiful thing because Kiibo himself experiences discrimination and bullying over his status as a robot. So, when he meets Kaede and discovers that she’s mute when she tries to sign to him or at least vaguely gesture to tell him that she’s mute (if she assumes that he doesn’t know sign), he understands pretty well and signs back to her. Which was an honest mistake–she’s mute not deaf–and she corrects this with her sign and a small smile. Flustered, Kiibo quickly apologizes, and Kaede knows that it’s an honest mistake. She’s actually really glad that *finally* someone wasn’t put-off after knowing that she has a disability, which leads to these two sticking together. They’re often seen during freetime and while Kaede practices on her piano in preparation for an exam.
I also imagine that they grow even closer together, occasionally flirting with each other casually. Although she’s a little “school girl shy,” Kaede flirts playfully with Kiibo…who seems to stumble around like a curious and equally shy schoolboy LOL. To be honest, Kiibo doesn’t know the first thing when it comes to flirting, so he looks up some cheesy lines on the internet that make Kaede cringe and giggle because they’re so silly. She’s grateful for Kiibo and is surprised when he makes the first move to ask her out and even ask to go steady with her. They became a cute couple in the classroom, spending lunchtime together and attending school plays, sports matches, and other festivities. Seeing them happy together also attracts the other students who were admittedly alienated from Kaede after finding out that she was mute and they didn’t know how to sign. Since Kiibo’s here, he teaches all of them to sign during their collective freetime or afterschool when they return to the dorms.