Carroll is a Water Elemental that Finn encounters in the episode "The Tower." She used to be a body of water, but after growing tired of people swimming in her, she evaporated. She at first mistakes Finn for one such swimmer, calling him a "real swim nut" after face-blasting him with her lightning gun. Carroll becomes nicer to Finn once she finds out he is having problems and is not a swimmer, and even offers him some advice. She lives in a cloud house that can be flown around.
Carroll appears again at the end of "Come Along With Me" as a Water Nymph. She can be seen drawing at the edge of a body of water in The Grotto with the ponytail Water Nymph behind her.
As a Cloud Person Carroll is a puffy, small, and has stubby limbs. She has cloud 'hair' that covers her left eye. As an Ice Person, she is made out of a solid angular block of ice. When she is shown as a Water Nymph she has the blue skin and flowing water hair as the other nymphs, but she wears a black one-piece swimsuit.
Carroll takes on a different form in each of her appearances, though she always has her "hair" combed over one eye. The three forms represent different stages of the water cycle, including gas/vapor (her cloud form), solid (her ice form) and liquid (her pond and Water Nymph forms).
In her last appearance, she is drawing a picture of a woman with a bow and arrow and pointed ears, referencing her remark in "Winter Light " that she draws elves. This elf drawing is seen inside BMO's home as "King of Ooo."
Water Carroll concept art by writer and storyboard artist Steve Wolfhard
Ice Carroll concept art by writer and storyboard artist Steve Wolfhard