Yang Xiao Long

Yellow is one of the main protagonists in RWBY. She has yet to make an appearance.

Speculation
The following is fan speculation, and is not to be considered canonical.

The lyrics in the song Red like Roses, along with Red's strong resemblance to Red Riding Hood, have led fans to believe that each of the main characters is based on a popular fairy tale. In the case of yellow, possibly Goldilocks or Rapunzel.

It is rumored that Yellow's weapons are guns, but we won't know that for sure until she's introduced...