Pen is a ballpoint pen (also called a biro or a ball pen) with a red shiny cap. She has black-dotted eyes, as well as a black mouth, arms and legs. The pen is located inside her, in the middle of her transparent polystyrene body.
As Pencil, Eraser and Cap reach the summit of the bookshelf in Time Wasters, they all meet Pen also standing on the summit. Cap runs off into Pen's arms and asks Pencil and Eraser what they came up for. After getting an answer, Pen asks if they came up here for the view of the classroom, which surprises the two. When Eraser Wash also came up the summit, she wanted Cap to come back, but Pen stated that she should see the result of her adventure first. After looking to the view for a while, Pencil asks how Pen came up here, much to Pencil's surprise, Pen says she came up via an elevator instead of using her bare hands.
Pen seems to be very joyful.
- Writing: TBA
- Cap: Pen and Cap seem to know each other very well.
- Pencil: Pen seems to know Pencil
- Eraser: Pen seems to know Eraser
- Eraser Wash: Pen seems to know Eraser Wash
- Pen resembles a BiC Cristal pen.