Possible Clues:

• Childhood playmate, perhaps
• Childhood companion depicted five times in this puzzle, thanks to some Down clues
• Invented playmate
• Any of the starred answers, some say
• Hobbes, in 'Calvin and Hobbes'
• Unseen playmate
• Childhood chum, of a sort
• Make-believe playmate
• Hobbes, to Calvin
• Childhood playmate