For this one, I’d imagine Keramon showed interest in Wallflower. Seeing the girl as a contradiction. Someone tired of being ignored but, going out of her way to erase the memories of herself. Before getting to know each other, Keramon acted on it’s own facination. Asking itself the question, what would happen if she wasn’t forgotten. While the memory stone works on a persons memories it can’t alter physical information. So Keramon would hack phones set reminders in peoples phones to not forget Wallflower, resulting in Wallflower making friends of her own. Despite her tendencies to eraese every little embarassing meetings.
After that they actually got to know each resulting in an actual friendship rather than, Keramon’s interest in her. Which lead to the Dark-Maildramon digivolution.
Decided to give Wallflower the Vital-bracelet digivice, because Keramon X looks terrifying so . As for why he’s her partner. I asked myself what digimon would get in Wallflowers way. As for why it can only reach the champion level. They’re friendship just started. Which is why I put her status as just Student.