Omnom88 I suppose you are using spoilers to make messages more compact? I'll do the same at least while communicating with you.
IIRC toro said it has to do with her status as a being born from a fairy tale. Other beings born from fairy tales, mainly madness spirits but also Vernai, don't change appearance as well except Red. Red might be a special case since 1) she comes from outside the garden and 2) she was created by Mary Sue but overcame the fate originally assigned to her by Mary Sue.
I'd like to agree with that, but knowing how SEN works and both Goose and Red Hood being made by Mary Sue it doesn't really adds up. Kuti also comes from outside the Garden and without invitation, but she has humanoid appearance and speech with high SEN. Though I was partially mistaken as well, since Blackwell is a human but SEN doesn't affect her appearance, so I must agree on Red Hood being a special case. Also Hein, Victoria and Miranda are not affected by SEN if I remember correctly.
I have my own interpretation on this, so correct me if I'm wrong.
When we first meet Red Hood, Grimm's mind was warped by TCO.
Jack mentioned The Crawling Little Girls right at the first encounter with Red Hood, which is suspicious in my opinion:
You must be registered to see the links
I can also add to that, that there are at least two cases where I think that we see a real Red Hood:
1) When we met Jack the Ripper. (Encounter that I mentioned before)
2) When we fight with Elizabeth.
Why? Because in these cases Red Hood doesn't become invisible with high SEN like red eyed monsters called "Love", if you translate that alien language.
In cases where Red Hood becomes invisible probably not Red Hood at all, but one of TCO's "eyes" so to say.
I do see a problem with one of the theories as it would be stupid for TCO to stop Grimm from reaching endings F and G where he finally meets him, because Red Hood is important for that, but maybe that only adds to his madness?