Replying...
Intro. You play as sonic and tail said he turned into a girl (context Tails as a five year old, although his final age ended up being eight. Miles "Tails" Prower[a] is a character created by Japanese game designer Yasushi Yamaguchi. Part of Sega's Sonic the Hedgehog franchise, he is the first character to consistently appear by Sonic's side in the series, appearing in nearly every mainline and spin-off game since his debut. His role as Sonic's best friend and sidekick is analogous to Luigi from the Mario series. The name "Miles Prower" is a pun on "miles per hour", a reference to the famed speed of Sonic the Hedgehog. Prower is a two-tailed anthropomorphic fox cub, hence the nickname.

Tail is a... GIRL!?

@Leo