Hako-Maro is Nekomata. She was Idora's pet cat when he was little.
A petit girl with white hairs, spotting two white cat ears on her head. She also wears a bell.
During her time when she was alive: She was a pet of Idora. Idora thought Hako-Maro was a boy due to the manly name (in Japan I think it's something related to manly, or this is what the manga says lol please comfirm).
After her death: She came back to life in form of a "spirit" just to tell Idora she's a girl. Idora immediately hugged her for he missed her and they lived together again.
Idora is the owner of Hako-Maro. She came back to "life" after her death just to clarify that she is in fact a girl all along. They have been living together again since then.
Initially, she views Kuro as a foolish and unintelligent dog but she later accepts Kuro and treats her as a reliable friend after their talk.