Artist: yuyu

OYP's resident artist. Known for the vibrant color schemes and high quality of the coloring. Has been inactive since 2011.

There's also a japanese artist also named yuyu and is using this tag too, but is unrelated to the resident yuyu. Started in 2014 but went inactive in late 2015.
Jp Yuyu: https://www.pixiv.net/en/users/10490811/artworks

Maybe both Yuyu should be separated into 2 tags one day. For now though, you can easily tell which Yuyu is the artist by the upload date (everything 2014 onwards is Jp Yuyu) or by the artstyle.