So there's a girl (kyu-kurarin) and a cute girlfriend tag for the unnamed girls presented in the songs, but I'm not sure about their utility. There are stuff like mika-chan and merry (readymade) but at least they were given a name. We do have the tendency to try and tag even the most minor characters, but this is a very small song copyright and she already is the only character of that copyright, without even a name. If we really think this is necessary then we have no excuse to not do the same to other song characters, which… Look at the amount of vocaloid songs on this site that features at least one original character.
And to be honest, I don’t even think the named character tags are more relevant. Mika-chan is fine since the song primarily features Kagamine Rin and Gumi, meaning she isn’t always seen in artworks of the song. But Merry (readymade) and Naima (usseewa) are the protagonist and the only character of their respective song, there isn’t much meaning to give them tags when the song tag already serves that purpose. We didn’t do it to all the other songs out there.