Piro's sketchbook
Piro's sketchbook contains a collection of his drawings, mainly of sad or troubled looking girls. He loses it together with his bag while knocking down a display in a manga-shop. It is picked up by Yuki who, impressed with the quality, declines to involve her dad in returning it. She then starts to annotate these drawings.
After discovering where Piro works, she decides she'll return it, although not until after scanning most of it. She does manage to return the sketchbook and vigourously defends his ability.
Piro felt the sketchbook was junk but felt its absence strongly. Yuki's annotations embarassed him, but Erika felt them insightful. There is a DPD showing one of the drawings with Yuki's annotations.
The sketchbook stays in Piro's bag until he goes to the Anna Miller's to give Kimiko a drawing (with annotation) of Kotone. Kimiko subsequently puts it on the fridge in Hayasaka Erika's apartment.
Yuki was inspired by the sketchbook to ask Piro for drawing lessons. Yuki is shown with her own sketchbook and pencils when she shows up for her first lesson, which Piro misses. She brings it again and shows it to Piro when the lesson finally occurs.