Miles "Tails" Prower is a two-tailed fox with the ability to fly by using his tails like helicopter propellers. He also uses them when running to keep up with Sonic the Hedghehog. Tails made his debut in Sonic the Hedgehog 2. He was created by Yasushi Yamaguchi, was intended to be his sidekick and has remained a staple of the Sonic franchise.
Like Sonic, Tails' real name makes reference to speed: Miles Prower is a pun on "miles per hour" (a joke apparently lost on the developers or translators of Sonic Chaos, in which his name appeared as "Miles Power").
As with the rest of the characters in the franchise, Tails' hand drawn design changed with the release of Sonic Adventure, from the design of Tails before the game in question.
Tails has the questionable honour of being one of the only non-Sonic protagonists in the Sonic series who is near-universally accepted by the fanbase, who often dislike newer additions to the cast.
An internal competition was held at Sega to come up with a new sidekick character for Sonic 2. The most popular entry was Yasushi Yamaguchi's sketch of a young two-tailed fox. Yamaguchi really wanted his character to be called Miles, but he was overruled and Sonic's new buddy ended up as "Tails".
Having been upset at the name change, Yamaguchi snuck the name "Miles" into the game's menu screen, and even all over the signage in the Casino Night Zone. In the end he won a compromise: Sonic's new sidekick was officially named Miles "Tails" Prower ("Tails" making the cut as the character's nickname).
Tails is partly inspired by Japanese mythology, which includes mystical fox spirits, known as Kitsune, which are magical beings that are able to assume human form. The older a Kitsune is, the more powerful it becomes, and the more tails it has. They are said to gain a new tail every hundred years until they've reached a maximum of nine tails. Kitsune were also the inspiration behind the Pokémon Vulpix and Ninetails.
According to ScrewAttack Miles "Tails" Prower is the number 1 sidekick.