Sealing Club Supernatural Entity File #8
The Bright Lady

Type: Youkai
Supernatural Abilities: Naturally intangible and capable of invisibility and flight. Capable of possessing humans.
Nature of Abilities: Innate youkai abilities.
Time and Place of Observation: Multiple incidents and locations.
Possible Weaknesses: Vulnerable to youkai extermination tools. The Bright Lady also depended on inducing some degree of apathy or hatred in her victims to keep them from resisting her influences. This could be countered to some degree by positive emotions, so they could be considered a countermeasure.
Intelligence Level: Human-level.
Size: Intangible, but human-sized.
Other Notes: The Bright Lady is the only one of the youkai from the incident who was able and willing to clearly tell us about her goals, so she's easily the one that I know the most about. Unfortunately, this means that I had to talk to her. She was open about her dependence on human despair and suspicion, and her actions were usually consistent with trying to maximize these at any given time. The only thing that hinted at motives beyond that was her apparent desire to control Kaguya Hourisan's (Entity File #10) body, which can still be viewed as acquiring a useful tool for furthering her goals.

Judging by my own experiences, I think that the Bright Lady's possession involved two different levels of control. At the most basic level, she was capable of controlling a victim's body. It was possible for a sufficiently determined victim to resist this influence, and her control seemed to work better when the victim wasn't aware that anything was amiss. So, she also subconsciously manipulated her victims, filling their head with thoughts and emotions that were designed to make the victim feel apathetic or positively toward the actions she made them take. She also displayed the ability to temporarily disrupt memory formation in order to make a victim perform actions that they couldn't even remember afterward.

Since the Bright Lady was capable of turning invisible and intangible, very few practical countermeasures existed. Mokou has told me about techniques that can be used to resist possession, but they require esoteric spiritual training. I suspect that a target with enough willpower could have outright resisted her control, but this remains purely theoretical.