Monstar is a minor boss in Paper Mario. Despite appearing to be extremely strong, Monstar's attack power is only one, and HP is only twenty. It attacks by creating a huge fireworks display that does one damage to Mario. However, Mario can do the Action Command to block and take no damage. After attacking, it asks Mario if he is scared yet in a shaky voice. Unlike other enemies in the game, the attacks Shooting Star and Star Storm do nothing to Monstar, because Monstar is made up of stars.
When defeated, Monstar is revealed to simply be a group of Star Kids. These Star Kids were trying to protect their home, Starborn Valley, from evil-doers like Bowser and the Crystal King. Since Bowser stole the Star Rod and these Star Kids could not rise up to grant wishes, they were stuck in Starborn Valley. So, the Star Kids had nothing better to do until Mario got the Star Rod back.