The same process as for the other characters. The only difference is they are based on established creepypasta or creatures from Japanese legend. Mikawa is a standard yuurei. Mieko-kun has the dominant features of a yuurei with elements of the fictional character his living counterpart was based on. (That's an easter egg/homage if anyone ever spots it.)
Yuri is a Japanese-style story about the relationship between women. Some are very pure "S-school" for example. Some are fetish. But most are about romantic or near-romantic relationships between women. The lesbian subtext is very common. Resent Yuri has featured more overtly lesbian themes.
The percentage of yuri that is fetish is about the same as the amount of romance that is fetish.
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