Kenna Yatsushiro

Yatsushiro is a tomboy who is considered rougher than the guys. She finds stairs a pain, so she jumps out of windows, and once when she couldn't figure out how to open a door, she destroyed it instead. Despite this, she does have a girly side, as she developed a crush on one of her fellow classmates. She has poor handwriting, but after a week of practice with Kouki Akune, her penmanship improves to a level where she is no longer embarrassed by it.

[Spoiler] Ten years later, Yatsushiro is a calligrapher. [/Spoiler]

(Source: Medaka Box Wiki)