The J represents Anthony's father, Agent Blaster.
The U represents Zero, Anthony's NetNavi.
The N represents Anthony's P.E.T.
The I represents Anthony.
The O represents Bass, Sidney's NetNavi and Zero's arch-nemesis.
The R represents Anthony's mother, Peggy, who is a version of Ms. Fieldmouse from a Portuguese remake of the Thumbelina storybook I found in the library and I also got that book when I was in elementary but I don't remember where I hid it.