As far as shorts go, this one was decent.
Story: There wasn't really much of one, the short being only 10 minutes long. As Marimo's classmates recount stories of times she saved them, a group of 5th grade deliquents enters the classroom and challenges Marimo to a duel. Having just heard her say she doesn't like fighting, Marimo's friends attempt to stave off the fight, but she ends up having to save them anyway. After defeating the main boss, she suddenly remembers that she has to water the school flower bed before the end of lunch, and her friends dash off to help her.
What I liked about the "story" is the martial arts philosophy it embodies. Nonviolence, reluctance to fight, efficiency and honorable methods when driven to a fight, and not basking in the defeat of your opponent at the end of a fight. [What I didn't like was how she had to let her male friend try to defend her, even though he didn't stand a chance at finishing the fight before she had to get involved. It smacks of a gender-role statement, although I guess it was in line with not taking the first shot at your opponent...?]
Characters: I thought the designs for the delinquents were pretty cool (but also nothing stellarly inventive), and the characterization was weak and stereotypical.
Overall: This show was fine for a short, but ultimately fell victim to the inherent limitations of shorts: simple plot and characters because no tiem for srs development.