Boobs of Steel simplified; A character is female, fights, and have large breasts. Tomoyo fits this description. I'm sure there's a character sheet of her somewhere.
Mind you, Boobs of Steel could probably be considered part of You Fail Biology Forever, since the only sensible reason to have a character with Boobs of Steel is to draw the Male Gaze. Is also part of the Most Common Superpower.
Hina is a pettanko, and the series seems to enjoy poking fun at this.