The Himarra Badge is a special item that is only found in Dark Cloud 2.
Its in-game description reads : "A mysterious badge that lets you change into a Himarra."
This special item is the first Monster Badge that you'll obtain in the game. It'll let Monica transform into a Himarra when in Dungeons' floors levels. This can be done by pressing R3 in Dungeons, like you would do for Max to use the Ridepod.
As a Himarra, Monica can safely approach the other Himarra monsters without the risk of fighting them. She is also able to talk with them. Doing so will be necessary as the Himarra will give to her a Sundrop in order to advance in the story.