Cloudy is one of Star Butterfly's summoning spells and magical friends. He is a living cloud that Star summons with the "Summoning Cloudy Charm" spell.
Cloudy typically appears as a sentient pink cloud with eyes (usually blue, sometimes green) and a mouth. When summoned while Star is channeling her magic negatively, Cloudy appears green instead of pink.
In "Royal Pain", Cloudy briefly appears among Star's various monster friends when she shows King Butterfly her bedroom. He also appears near the end of the episode, serving as Star's transportation.
In "Marco Grows a Beard", Cloudy once again acts as transportation for Star when she goes to school, and he appears to be in agonizing pain when Star poofs him away.
In "Wand to Wand", Cloudy appears green instead of his usual pink when Star channels negative emotions while casting Summoning Cloudy Charm. While in this state, Cloudy is sluggish and carefree, creating a big mess around the Diaz Household and not caring. When Star tries to zap him back to normal, he gets mad and grows into a furious storm cloud, impervious to many of Star's magic spells. It is only when Star channels her positive emotions again that her magic finally turns him back to normal.
In "Face the Music", Star conjures up Cloudy twice; once to help her get away from Ruberiot, and the second time so she could avoid Marco after her crush on him was revealed by Ruberiot's song.
On most occasions, Cloudy appears bright and smiling, happy to help Star in any way he can. When influenced by Star's negative magic, however, Cloudy is lethargic, carefree, and somewhat passive-aggressive. He cares little about the consequences of his actions or about Star's wishes.