I searched the forum for this, but couldn't find any reference. I guess this character is from a manga called Monobeno: Sonogo no Natsuha no Nanatsu no Natsu. I think this might be from a game? This is a wiki entry I found if anyone is interested. She apparently becomes an adult overnight or something.
https://ja.wikipedia.org/wiki/ものべの
Anyway, I thought the pic was cute!