Katsuo Atso's Death

Katsuo Atso died at age of 9 caused by magic, ball explosion, that caused huge change into Usagi's life.

It happened when Usagi first receive her mission partnered with Katsuo Atso, they were send to isolated laboratory of where illegal man-eater monsters were created for war. Their mission was to capture or destroy the data of the project to prevent anyone trying to retrieve it who may want to use it for themselves.

By the time, their mission was complete a monster came in from nowhere.

Katsuo became her shield and died there.