Piro's black messenger bag has been with him since the first strip. (The backpack Largo wears in that strip disappeared after Largo was hit by a bus.) Piro later lost the bag, leaving it (and his sketchbook) in Yuki's possession. She admired his drawings and left comments on every page of the sketchbook. Her interest in Piro's drawings soon developed into a crush, which began affecting her schoolwork and social life. Once she plucked up the courage to go and return the bag, she also asked him to teach her how to draw. Thus far, only one lesson (interrupted midway through) has taken place due to a combination of circumstances. Following said incident, Yuki accidentally took Piro's bag with her when she left, revealing her kleptomaniac traits.
The bag is a YakPak Medium Flapdoozy, identical to one the artist, Fred Gallagher, owns. The real-life bag is large enough to carry several small notebooks and a textbook, includes several internal and external pockets, and is reputed to be very rugged. Megagear sells a version of this bag branded with the Megatokyo logo; this version also includes a pocket for a cellular phone, rectifying a criticism Yuki made of its design.
Some confusions have arisen recently with Yuki's recent appearances, in which her bag plays a role. This is not Piro's, as Ping returned it at the end of chapter 7. (Note that by chapter 8's beginning, Piro has his cell phone back.)