It took me a while but I think I found one answer, or rather 3 answers. I got a few things wrong, we don't look for someone pre-pubescent, but instead someone in puberty or after since the maturing of the genitals makes the labia majora puffier. Also after 25 the skin starts to get older and lose tautness. So everyone over 25 physically is out. Aswell all fairies and animal youkais that aren't mammals since the first are questionable to ever sexually mature and the latter I'm unsure what they have between their legs. Also I removed the sluts like Sanae.
So next we go by skin tautness and blood circulation. Generally a few things are bad. Unhealthy lifestyle (smoking, drinking, junk food), UV-light for skin, stress, few physical training, hot or cold weather. So we remove everyone that is physically frail, lazy, drinks (oni, heavy party goers), eats unhealthy, is stressed, lives in hot (old hell, although hot baths help circulation) or cold climate. I also removed the soldiers/guards because they likely have stress or drink (they always drink).
Now I make a break between two different assumptions, one is that lolis aren't sexually mature and the other that they are.
If the aren't, I'm down to Hina, Kogasa, Nue, Kyouko, Sekibanki, Seija and Nitori. Hina might be actually too old and Nues genitalia is in question, additionally Kogasa might be malnourished due to her being bad at surprising people. So between Kyouko, Sekibanki, Seija and Nitori, I would give it to Nitori as cucumbers help circulation and she keeps herself well hydrated.