Many thanks go to our very clever, resident Nintendo Papercraft designer, ddi7i4d, for creating this wonderful gem.
Boos were originally introduced in Super Mario Bros. 3 (under the name Boo Diddly!), and have since then been a regularly recurring enemy in many of the Mario games; they have even appeared as a playable character. As ghosts, they are able to pass through solid matter. However, they are shy, and will cover their face with their arms and become immaterial if they are directly looked at.
The brilliant thing about this particular model is the fact that it’s almost impossible to see from this particular angle (see image).
Edit: Super clever mr. ddi7i4d has just made an alternate version of boo – Visible Boo – just incase you feel like mixing and match!