Reki

Reki's original name is unknown, but she was a Sentient Yōma who despised her mindless brethren for being trash. She came to Earth in order to exterminate the mindless trash for causing problems.

She appeared during the time when Hanzō and Kurokage were young, though she apparently did not directly meet them. She did hear about their exploits, however.

Reki ultimately ended up joining a team of shinobi and helping them exterminate the mindless Yōma. However, once the mission was completed, the shinobi backstabbed and murdered Reki.

Only a single young shinobi girl did not go along with the order. She had never wanted to be a shinobi, but was forced into the life because she had a talent for it. The other shinobi murdered her as well for standing against them.

As she lay dying, the human offered her body to Reki so that Reki could take revenge for them. Reki could have saved them both in the one body, but the girl chose to die rather than continuing living in this world she'd grown to hate.

Reki, now weakened by having to inhabit a human corpse instead of her original body, now carried a hatred for shinobi that equaled her hatred for mindless Yōma. She saw them both as nothing but worthless trash to be used and destroyed as she saw fit.

Now Reki is using Senki Shu to further her plans of destroying both the mindless Yōma and the shinobi. She has no qualms about mind controlling and sending as many of either group out to their deaths.

However, she has nothing against regular humans and will not harm normal civilians or former shinobi that quit the life.

The human corpse she inhabits seems to have it's age frozen at 18-years-old, presumably the age the shinobi girl was when she was murdered.