In Speed Highway, several billboards can be found depicting an unknown female character. In 2020, Satoshi Okano, a former designer at Sega who worked on Sonic Adventure, revealed that this was a scrapped spider character he designed with the prototype name "Spider Girl", and posted an old sketch of the character alongside a sketch of another scrapped character, a bat named "Fly the Vampire". In 2023, Okano posted an inked version of the original Spider Girl sketch from the same time period alongside several inked sketches from the development of Samba de Amigo. Spider Girl's appearance on the billboards in Sonic Adventure suggests that the character lasted far enough into the game's development to receive a 3D model, but the character was ultimately scrapped for unknown reasons, and this 3D model has yet to surface on its own. It's unknown what role either Spider Girl or Fly were intended to have, though fans have noted that Spider Girl has some similarities to Rouge the Bat from Sonic Adventure 2, namely that both characters have large bosoms and prominent heart-based/romantic imagery.