So I think the most prominent thought probably is the in-anime explanation where the masu assumed the form of Ryo-ohki's wishes. Ryo-ohki wanted to help Tenchi.... (although I don't recall it wanting to be a humanoid girl although that is certainly a possibility.) I take Ryoohki's love for Tenchi and entire personality comes from Chibi the cat which he nursed back to health in the summer before he freed Ryoko.
But if she wanted to be humanoid, what was her template? Doesn't really look like her "mom" or sister. I didn't really think about it until I came across some Japanese fan art of the Masu.... and went, hmm.... that looks like Naja...... a bit exaggerated towards her compared to the anime but not by much. Also the back of Masu's hair compares favorably with Kagato's when it was unleashed from its hair bind.
So was Naja the template for this Masu, or was Naja somehow Masu herself? I doubt there will ever be an answer, just a random thought about one of the most mysterious women of TM.
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Very interesting observation...
Never really thought of who humanoid Ryouohki looked like before, could be a form of genetic memory.
Could be that the Masu have been around for a long time and remember Naja. Perhaps Naja was the one who originally was interested in them?
It makes sense. Naja/Kagato would have had a hand in the creation of Ryo-Ohki