I did a little investigation; I saw in the source a "Weirdly worded" tag named as "yamashirorenn" (山城恋)
So I went and look at the serie's info on MyAnimeList
https://myanimelist.net/manga/116880/Mato_Seihei_no_Slave/

I went under "Characters"
https://myanimelist.net/manga/116880/Mato_Seihei_no_Slave/characters
And found a character whose name is suspeciously similar to the "Weirdly worded" tag: "Ren Yamashiro"
https://myanimelist.net/character/190004/Ren_Yamashiro

The name reported also matches on Pixiv's post title (山城恋 vs 山城 恋 on Myanimelist site)

Looking at the only picture, it appears she have the same hair ornament that also appear in this artwork -- therefore I conclude that this character is in fact Ren Yamashiro o.o!

*adds character tag*

On Danbooru this character is named yamashiro_ren. But there are barely any post of her.

All my investigation work for nothing :C

*sad trumpet noises*

Asian names must be tagged as family name + first name (yamashiro_ren, not ren_yamashiro).

Marona762 said:
All my investigation work for nothing :C

*sad trumpet noises*

You tried, that's something I did not do.