U-Packs (Pronounced "UU Pakku" in Japanese) are flying, animated cardboard boxes. They often carry around yukkuris or other objects. Sometimes they do it for free, other times they demand payment (usually given as food).
Some artists have drawn variations on the U-Pack, for example, the "amb~u~lance".
They have bat-like wings like Remirya, and it may be that this yukkuri's catchphrase of "Uu!" inspired their creation.
They do not seem to attack yukkuris, and are generally friendly with all sorts of yukkuri. However, several comics exist of them being attacked by Kimeemaru for unknown reasons, and it's common of them to play pranks on yukkuris. They have also been known to suddenly open up at the bottom while in flight, dropping any yukkuris on board. The cause of this is the yukkuri "refreshing" itself inside the U-Pack, or just bad luck.
They have been depicted as being born from eggs and living in nests. For food, they appear to enjoy most plant-based food items as well as junk foods (such as candy, chips, etc).
Within the fandomEdit
While popular, U-Packs are slowly fading out of popularity with very few artists portraying them in any work. This may be due to the more "realistic" nature of the current yukkuri fandom (focusing on cities and Japan rather than forests and Gensokyo). Regardless, there is a small section of the fandom that still holds onto U-Packs.
The name is a pun based on "Yuu-Pack", a brand of shipping supplies in Japan. (The original is itself a pun on the English word "you" and the Japanese 郵 (pronounced "yuu" in compounds), meaning "mail".)