Marie was a grey cat owned by an old lady. She is talked about in episode 36. She got wet one day, so her elderly owner thought to dry her off in the microwave. She lived 2 years in total.
Later, while visiting Marie's gravestone, the old lady was visited by Kid, who had become a lawyer. He convinced her to sue, and won.
This tale was told to Pierrot by Don. This story is the reason Pierrot hated microwaves for many years. He was about to penalize the Japanese team for this reason, but Shachihoko claimed the entire thing was a joke.
This may have been a lie so that he could keep Kazuma in the competition and honourably compete with him. After all, Kid was real, and Don never spoke to Ryo who knew kid, so it was probably a true story. Poor Marie-chan.