Della River is a ghost girl who appeared in the fifty sixth Goosebumps book The Curse of Camp Cold Lake.
History
She thought that she died from drowning, that's why she's in the water, Sarah met her while planning on making the camper's feel sorry for her. Della wanted a new friend in the after life, she wanted Sarah, but then it is reveled that she didn't die from drowning, she died from a poisonous snake bite.