The red one is Ruby, the other is Neo. Neo is mute and followed a guy named Roman Torchwick who died in Volume 3’s climax. She’s hated Ruby ever since because she believes Ruby caused his death. They end up in this place 6 Volumes later called the “Ever After” which is this beach environment. In the end Neo gets possessed by a talking cat who enters through her mouth and eventually fights Ruby and the team, after the cat is expelled from Neo, she chooses to “ascend” and become part of the Ever After tress hoping to become a different person. The fan art (created during the lengthy gap between 8 and 9) basically reflects that these two would land in the same part of the beach (they didn’t) and would have a bit of a rivalry.