I've always seen Yoichi as having a childish crush on Mikan, so I tried to convey this in a small way with this idea. I hope you can see it.
Mikan.
It sounds familiar. Enough that he's sure he heard it before. He asks Nastume-nii and he says Mikan was the girl who used to hang around with them sometimes. He somewhat remembers that—enough that he can say he knows her brown hair and brown eyes.
There's more he knows, but the dark-haired nearly teenaged boy looks pained, so he goes to ask Natsume-nii's best friend, Ruka-sempai. He says Mikan was a pretty girl, cheerful, but not to an annoying extent (at least not enough to make you want to strangle her).
Ruka didn't tell him everything either, but the already sensitive boy looks ready to cry, so he goes to Hotaru-sempai, hoping she would know something. She says Mikan was an idiot. After a pause, she adds that despite her hatred of math and idiotic moments (shouts, "I get it!" ten minutes after everyone else) she makes very good grades because she cares enough to study.
He frowns because these scraps can't honestly piece together a girl. He walks around campus and finds a picture. It all makes sense now, because she is Mikan.
He sticks it in his bag and never says anything because he wants to remember her face on his own now.