True to her name, Grouchy is not a happy, or nice, pony. She likes to complain a lot and is constantly mean to other ponies, constantly insulting them, or by harassing or playing pranks on them.
She holds a disgust for Hearth's Warming and all that it stands for, and makes it her own goal to ruin it every year! D: She does this be sneaking into everyone's houses when they are sleeping, and stealing away all the gifts and decorations. What a meanie! :c
Despite her attitude though, there are a few little ponies who think that she's not all that bad inside, and could be nicer if there was someone there to be friends with her.