Kusanagi maru

About
The spirit of the negative Kusanagi. She is responsible for the sword's anti-duendy capabilities as she sucks up the jaki with every hit. She is also quite good at detecting jaki and ki. Her only other capabilities are annoying Shura. The sword was apparently "given" to Shura by Deirdre.

She isn't actually evil despite her liking for death, destruction and the macabre in general. She inhabits Shura's body and he can seal or unseal her at will, perfect for when she gets too excited. Being a spirit, she can only touch things of a similar nature like objects from hell.

The sword (and spirit?) came into existence as part of a spell which would make the Kusanagi (one of the 3 sacred treasures) more positive. This meant the negative was to be transferred into the other sword.

In the battle with Fomoire, she was picked up by Karin which sent her into a confused state as she did not know what to make of her ki. This made her completely useless against a duendy whose head couldn't easily be cut. Because of this she dislikes Karin quite a bit.

Badb in her empowered state broke the Kusanagi which killed her. She was brought back by a somewhat reluctant Astaroth who put her in the reforged Kusanagi made by Arima. She got significantly stronger and more knowledgable about things, but her personality did not get fixed as she immediately insulted Astaroth.

In the battle against Nero it is shown that you can enhance her with magic, which she says she can feel unlike the enchanting Arima did on just the sword. This is used to electrify the sword and damage Nero. A similar spell was cast on her to suck more jaki from Deirdre which showed that she can suck too much in one go.

Trivia

 * The sword itself is actually 120cm
 * If Shura dies, she also dies.
 * The spirit is actually vampiric in general, but seems to prefer jaki
 * It is described that she is after emotions, not blood in particular.