Wannin!

Wannin! (ワンニン！) is a manga series by Sora Hoonoki. It was serialized in ARIA.

Plot
After 10 years of training, two young shape-shifting dog ninja are told that they're ready to be tested in the field. They find their new owner, Mitsuyoshi, who takes them in on the assumption that they are simply lost doggies. He gives them new names--"Hanzou" and "Hadzuki." The two dog ninja are supposed to protect him from any other demons, but they must not let him learn his new dogs' true identities. Unfortunately, Mitsuyoshi met Hadzuki in her human form very early on, when she rescued him from a couple of demons. Now she's got to keep her human and dog relationship with Mitsuyoshi separate, something not made easier by constant demon threats and her growing attraction to him.