Why the Chimera isn't a ghost entity (alias a "Yurei").
The in-game Kollektadex categorizes the Hex Maniac as "The Yurei", which is a broad Japanese term for what people in the West would call a ghost.
A yurei is often associated with spirits which were wronged for whatever reason, when or before their former living selves died. Because of this, they are malicious entities inflicting harm on those that have the misfortune to encounter them. Due to this categorization, one would assume the Hex Maniac is the main antagonist who controls the Chimera and whose goal is to harm and kill Serena. The centipedes (bugs, whatever you call them) would be just a creepy detail without any further importance other than representing the yurei thematically. Like a sign of decay, fear (entomophobia) or death.
However, if you look at some details shown during the gameplay, it turns out this categorization is rather misleading, if not false.
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1)The Hex Maniac isn't in charge, she's puppeteered by the bugs (I'll call them parasites from now on). If she were a yurei, it should have been the other way around, as she's supposedly the "main character". When Serena manages to crawl out of the basement and escape, the Chimera screams "help me".
2:59
https://www.youtube.com/watch?v=uhnnTM_ftVU
This is something out of character for an entity that has been depicted as an inhuman and predatory monster so far. Sure, maybe it's another attempt by the yurei to deceive Serena, but given how the Chimera is able to teleport, it wouldn't even matter if Serena stopped, since she's injured and already done for; there's no reason for the Chimera to say something like that, as giggling or some other deranged dialogue would have been more in line with its nature. Unless, the Hex Maniac is actually a host which manages to briefly break free from the parasites' control and attempts to communicate. One could claim it's still a yurei which begs a potential (terrorized) savior for help to release it from its torment, but as we see later, the idea of an unfortunate woman infested by parasites is more plausible.
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- The Chimera is photosensitive, as it gets stunned when Serena uses her camera's flashlights against it.
https://i.redd.it/d7dfdcws54kh1.gif
https://i.redd.it/1d6hguwt54kh1.gif
It's an odd weakness for a spirit entity to have, considering ghosts are usually not described as being weak to artificial (flash)lights (contrary to sacred light that is). It would make perfectly sense though if we consider it to be a weakness of the parasites. We know that the parasites hole out the eyeballs of their hosts in order to see (not explained how and why that is). Their photoreceptors might be sensitive, so a sudden bright light can be enough to temporarily stun them. It wouldn't make sense for a yurei capable of all kinds of supernatural powers (like making solid objects disappear or teleporting) to be so relient on its creepy accessories. Unless again, it's not a yurei and the parasites are the one's controlling the body, with their weaknesses defining the Chimera.
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- The steps describing how to prepare a new vessel for transfering the Chimera are meant for the parasites, not a ghost entity.
https://i.redd.it/e9nnwn2p24kh1.gif
>Step 0: 移植を開始するには器が必要です。適切な器を見つけて。
Step 1: 胴体を真ん中から切り開き、胸骨と臓器を露出させる。
Step 2: 腹部の臓器を取り除いて、居住空間を確保する。
Step 3: 肺と心臓を取り除いてください。彼女が呼吸していないことを確認してください。
Final Step: CHIMERAを移植する。
>(Translated with ChatGPT, so bear with me for potential errors.)
>Step 0: To begin the transplant, you need a vessel (or host/body). Find a suitable one.
Step 1: Cut open the torso down the middle, exposing the sternum and internal organs.
Step 2: Remove the abdominal organs to make room (literally, "secure a living space").
Step 3: Please remove the lungs and heart. Make sure she is no longer breathing.
Final Step: Implant (transplant) the CHIMERA.
>Note by ChatGPT: 居住空間 (kyojū kūkan) literally means "living space" or "habitable space," implying the body is being hollowed out to house something else.
It doesn't mention anything about possessions or whatever a ghost would do to "occupy" a human being. Spirits are incorporeal, they don't require space. They usually also possess entire bodies or objects, not parts of it. Though one could claim there are specific crucial organs of the human body that this particular yurei would need to get rid of in order to claim the body as its own. Curiously enough, there's the mention of "making room" or creating "living space", which not only opposes the needs of a ghost (why would something dead require living space?), but also makes more sense if it's meant for actually living creatures, such as the parasites. "Implanting" or "transplanting" sounds also odd for describing the act of a ghost possessing a living being, unless it's meant in the literal sense of transferring the parasites from one body to another, as seen in the true ending.
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- The Chimera has ghost powers not because it's a supernatural spirit entity, but rather the Hex Maniac already having ghost-like powers to begin with. To explain this, one only needs to point out that some humans in Pokémon have superpowers, similar to pokemon. If we look at certain trainer classes such as the Psychics, we'll notice that these trainers are usually shown using psychic energy clouds to make Poké Balls levitate, proving their psychic abilities. One could classify those as Psychic Types.
Other classes like Channelers or - you guessed it - Hex Maniacs are often shown to have connections to the spirit realm. Meaning it's not out of the blue to assume they have paranormal abilities, which one could usually find in Ghost Types. When the parasites take control over the Hex Maniac's body, they also gain access to her powers. With that, the title "Yurei" would merely describe what kind of infested host this Chimera is and what threat is poses (similar to how different kinds, subclasses or variants of zombies or other monsters have categories applied to them).
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- A yurei is a concept that is bound to a specific person. If you refer to one particular yurei, you are talking about this one specific (deceased) person. The curse surrounding the yurei represents a particular soul and is so to say personalized. With the parasites switching hosts, this connection is lost. As seen in the true ending, the Hex Maniac is no longer under the parasites' control, and her body is finally just a regular corpse.
https://i.redd.it/yxmvbqle34kh1.gif
This automatically means that the Hex Maniac wasn't a yurei to begin with. The moment the parasites abandon her body and seperate from her, she should have stopped existing as a yurei and so the parasites as the yurei's supposed accessories. Yet they don't, meaning they are the entities that created the Chimera in the first place. (Ironically, this one moment in the true ending could have be the origin story for a yurei, if we were to assume the Hex Maniac's corpse wasn't buried and unceremoniously left to rot in an open field).
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In conclusion, the term yurei is incorrect (so is the term "chimera" as well, as mentioned here). The Chimera isn't a spirit, but rather a person controlled by parasites that hijacks their host's body and abilities. With that, the Chimera is both easier - and also more difficult - to deal with. Easier, as you aren't dealing with some overpowered eldritch horror, but an entitiy that is bound to the rules of the Pokémon. More difficult, as the threat depends on whatever the parasites are infesting. We've only seen an infested Hex Maniac in action, so imagine what other Chimeras, such as pokemon or other people, might be capable of.