SEN is a katakana wordplay on insanity in japanese. Its also a bloodborn reference to the "Insight/Madness" mechanic.
100 SEN means 100 insanity/madness.
0 SEN means 0 insanity/madness.
You could also call it akin to constant brainwashing due the H route with GG explanation
Some people appear both at 100 & 0 SEN because these are their real body`s.
Some disappear because they dont exist or are fairy tale concepts/spirits.
Ultimately SEN boils down to Grimm`s perception of the world around him.
Grimm either sees for what Wonderland truly is at 0 insanity.
Or what Grimm sees with a wraped mind at 100 insanity.
Though it is true that SEN shows how much SEN
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/MUDNES (madness) the main character has, this mechanic does not follow one single rule, so it's not easy to say what exactly SEN is.
Basic reasoning:
The reasons for speculation are contradictive effects of SEN on the visuals and gameplay.
1) For example it works correctly if we are talking about Star Spirits, an equivalent of the Lovecraft's Star Vampires. They are completely invisible to the human eyes while being visible when they are full of victim's blood. In the game they are almost invisible at 0 SEN, which makes sense. It suggests that when SEN is less than 1, Grimm sees what a normal human would see.
They attack Grimm regardless of being visible or not, unlike red eyed creatures called "Love" if you translate their name. They attack him only at 0 SEN for some reason. Same as Her in the Slaughterhouse. It suggests that certain creatures can feel Grimm's SEN level and perhaps asking for attention.
1.5) Red Hood in this case follows weird logic. At the first encounter you can see her in both 0 and > 0 SEN mode, while any time after that you can see Red Hood with the same SEN you can see "Love" creatures. It suggests that you saw real Red Hood only once.
1.55) It also weirds me out that with SEN more than 0 Red Hood looked like a beast, while many other characters changes into beast form at SEN less than 0. It suggest that Grimm's perception is inverted with SEN more than 0, with a few exceptions like Mabel, Node, Prickett, TCO and almost anything you see in the DLC3.
1.60) Why? Because some illusions are too strong to be affected by SEN. It is my personal explanation.
2) 0 SEN make corpses turn invisible, while in reality they do exist, same as Star Spirits. It suggests that many creatures were changed by gods to make them close to gods' Outer nature so to say. But it has a problem, since items Grimm picks up from the invisible creatures do not become invisible regardless of Grimm's SEN level.
3) It makes certain doors "appear" and "disappear", like the one in the Rabbit Hole, Slaughterhouse and door to DLC3 in the Carroll's River. It suggests that Grimm can't use what he can't see.
4) It change the
meaning of words Grimm can perceive with different amount of SEN, like the statue of Wolris, Jack's words when we firstly encounter Red Hood, sign at the Park of Fog and translated gibberish Griffy's words at the bridge. It suggest that Grimm hears and sees the opposite of what characters originally meant. Sometimes meaning can change drastically depending on SEN level, like on sign at the Rabbit's Hole elevator, Snowmen and Madonna in the Box.
5) Sometimes SEN gives unpredictable effect.
5.1) It almost doesn't affect Goose and I can't explain that, but it gives a simple phrase in her dialogue,
suggesting that she remembers what Grimm did.
5.2) When SEN is more than 0, it can make Grimm perceive not alive creatures as alive, such as one in the Mental Ward, Ox Ward University and Mabel's clocks.
It suggest that Grimm can hear, see and understand changed inanimate objects, if his SEN is high enough.
5.3) Certain locations like Witch's House and Hotel Poseidon perceived very differently with SEN more than 99, making Grimm vivid dream some memories and characters than don't exist anymore.
It suggests that Grimm's memory become too vivid when he is completely SEN. Though the result of flashback in the Hotel Poseidon is weird.
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, but instead it lowered to 0,
as if SEN was deleted artificially by the gears.
Conclusion:
It would be easy to say that when SEN is more than 0 Grimm's mind inverts original meaning of what he sees and hears, while also seeing what a normal human wasn't suppose to see, but as we can see SEN is not as simple as it may look.
