A Guardian (秘伝動物, Hiden Dōbutsu, Literally meaning: Secret Animal) is the animal used as the foundation for a shinobi's Secret Ninja Arts.
The animal of choice can be real or fictional, and all that matters is whether or not the animal of choice matches the person's innate characteristics or in other words "compatibility. It is also possible for a person to have more than one animal.
Shinobi drawn on the power of nature to use their supernatural techniques. Secret Ninja Arts, in particular, allow a shinobi to use his or her spirit animal avatar to unleash extra-powerful attacks. These animals are called guardians.
The degree to which a guardian actually manifests depends on each shinobi's abilities. Some can summon imaginary animals, like Ikaruga with her phoenix, while others summon ordinary animals such as Hibari with her ninja rabbit. Still others merely borrow a certain animal's trait, as in the case of Asuka and her frog-like jumping ability.