The Sonic universe refers to the Smash Bros. series' collection of characters, stages, and properties that hail from the world famous media franchise owned by Sega and centered on its company mascot, Sonic the Hedgehog. This had been easily the most anticipated new franchise for inclusion in the Nintendo-based fighting game series and is considered the biggest "rival" franchise to Mario. Its logo is a silhouette of Sonic's head, similar to the logo of Sonic Team.
 Franchise description
By 1990, Sega had released its 16-bit successor to the Sega Master System, the Mega Drive, worldwide, and it was renamed the "Genesis" for its North American release (literally because Sega had been unable to secure legal rights to the Mega Drive name in North America). Nintendo's Mario franchise was at the height of its worldwide commercial success, with the recent release of Super Mario Bros. 3, and both the 1991 release of Nintendo's own 16-bit system, the Super Nintendo, and the launch title Super Mario World were coming up fast. Sega made a conscious effort to compete against Nintendo's powerful brand with a reliance on a new mascot for both the console and the company itself, one that would help sell systems, and began development of its own platformer in April 1990. The game placed an emphasis on horizontally-lengthy levels that could be navigated with a player-character that could run, roll, and bounce through at a high velocity, with movements that were dictated by elements of momentum-based physics. The screen famously scrolled as fast as it needed to keep up; it was a very technically difficult process to create the game's graphics engine so that it could allow this speed without sacrificing graphical clarity. The end result was the worldwide debut of the eponymous character Sonic the Hedgehog, whose game was released in mid-1991.
The game Sonic the Hedgehog was both a critical and commercial success, and greatly increased the popularity of the Sega Genesis in North America, especially when it replaced Altered Beast as the game bundled with the console. It was to the point that the Genesis outsold the Super Nintendo nearly two-to-one during the 1991 holiday season. The game - which would eventually sell 15 million copies and become the best-selling Genesis game in its lifespan - is credited as single-handedly changing the course of the 16-bit generation of video game consoles and providing a legitimate alternative to Mario in the eyes of many consumers, as well as influencing the development of various 2D video games in the following years (many of which were centered on their own mascots). For the first time since 1985, Nintendo was briefly overtaken as the leader in the console market. This subsequently gave way to one of the most endearing video game rivalries in the industry's history, the "Console Wars" of Nintendo's SNES versus Sega's Genesis, which were symbolized to some degree by the image of Mario versus Sonic. The stiff competition between the two 16-bit consoles arguably stimulated both companies' stables of video game franchises, and also spawned some famous historical pieces of marketing; the SNES was technically superior in every way except processing speed, and Sega capitalized on this by advertising how "Genesis does what Nintendon't" by having faster speeds in its games, but in place of reciting this technical difference in commercials, Sega marketed the Genesis' faster processing speed as "Blast Processing".
As would be expected, Sonic starred in many high-profile followups, not just on Genesis, but on each of Sega's follow-up consoles and handhelds, in a similar pattern to Nintendo customarily releasing a game centered on or involving Mario at or near the launch of each of its own consoles or handhelds. Sonic the Hedgehog 2 was received as a marked improvement over its predecessor and is regarded as one of the best games on the Genesis, while Sonic & Knuckles became the only Genesis game that could have another Genesis cartridge inserted onto the top of it (marketed as "Lock-On Technology"), and this was used to turn the game into a physical expansion of the previous game Sonic the Hedgehog 3 when that was inserted. Following the Genesis, however, Sega's fortunes as a competitor in the console market began to buckle in as the company began to make a series of questionable decisions for its future hardware plans - two expensive, separate add-ons for the Genesis, the Sega CD and 32x, failed to attain their own significant libraries and stretched the company's resources thin, and what was intended to be the appropriate next-generation followup, the Sega Saturn, was infamously released four months earlier than anyone anticipated, including other game developers, in an attempt to gain an "edge" over Sony and its recent PlayStation console. Despite Sonic continuing to star in games intended to sell these consoles - Sonic CD for the Sega CD is debated to be one of the best Sonic games in the series' history - Sega had soured many on its own company brand.
Sega's final console to be released, the Dreamcast, was released in the West on 9/9/99. As the first console of the sixth generation of video game hardware, it was widely hailed as ahead of its time, both technically and for its pioneering of online console gaming, and is retrospectively agreed to have been a much better-thought-out and executed system by Sega. The launch title that became the undisputed killer app for the console was Sonic Adventure, the first game in the series to feature free-roaming three-dimensional gameplay. It received glowing reviews for its successful transition of the fast Sonic style into three dimensions and became the best-selling Dreamcast game. But despite this success, Sega was in dire financial straits because of its failed hardware plans the previous years, and when the other high-profile consoles for the sixth generation were unveiled - the PlayStation 2, Xbox, and Nintendo GameCube - Sega discontinued the Dreamcast in the West early March 2001, a year and a half after its launch, and withdrew from the console hardware business altogether and restructured itself as a third-party developer.
As a third-party developer, Sega was now in a position to release its titles for consoles that were formerly its competitors. The first releases of Sonic games on systems owned by Nintendo, its former arch-rival, were widely publicized; these included ports of both Sonic Adventure and its sequel on the GameCube, as well as the Sonic Advance subseries on the Game Boy Advance. The Sonic franchise settled into a more stable release schedule, and the series continually branched out into a variety of genres for all of the competing platforms, though there were several releases that were met with notably mixed or negative reception; Sonic the Hedgehog, for the PlayStation 3 in 2006, was notorious for technical issues (resulting from it being rushed into the system's launch window), control problems, and a slant towards story and characterization that were negatively received as uncomfortably melodramatic, while Sonic Free Riders served to highlight control issues with the Kinect peripheral for Xbox 360. Nonetheless, there have also been commercially successful forays back into the series' platforming roots, such as with the episodic Sonic the Hedgehog 4 releases on modern consoles' downloadable services, as well as Sonic Generations for Nintendo 3DS and other companies' consoles, which acknowledges the classic and modern design incarnations of Sonic and his games' 2D and 3D level design styles.
The legendary scenario of former video game arch-rivals Sonic and Mario crossing over in the same game was finally realized in late 2007 with the release of a game that pit the two against each other in a very unexpected scenario: a sports party game titled Mario & Sonic at the Olympic Games, where selectable characters from both the Sonic and Mario franchises compete against each other in all of the sporting competitions associated with the 2008 Summer Olympics held at Beijing, China. This became a subseries - developed by Sega, published by Nintendo, and officially licensed by the International Olympics Committee - that would regularly release new installments for both Nintendo's consoles and handhelds at a biennial rate to correspond to each of the subsequent Olympic seasons - the 2010 Winter Olympics held at Vancouver, British Columbia; the 2012 Summer Olympics held at London, England; and the 2014 Winter Olympics held at Sochi, Russia. Meanwhile, Sonic was chosen as one of the first two third-party characters to co-star alongside Mario and many other Nintendo characters in the Super Smash Bros. fighting game series, in Super Smash Bros. Brawl.
Sonic the Hedgehog games are set in an Earth-like world populated by wildly anthropomorphized, colorful animal characters, but unlike Star Fox, humans feature in this world as well. The main character is Sonic, a blue hedgehog with an attitude and a pair of striped shoes as his only articles of clothing, and he has the capacity to run extremely fast and curl up into a rolling sphere. In a format similar to both Mario and Mega Man, Sonic's most persistent adversary is a mad human scientist bent on world domination named Dr. Ivo Robotnik (most commonly called "Eggman"), and Sonic speeds his way through levels to defeat him in his various combat machines and free innocent animals that have been transformed into robots by Eggman. Powerful jewels called Chaos Emeralds are both what Eggman covets for his schemes and what Sonic must collect to temporarily transform into an invincible "Super" mode that can help him defeat Eggman. Over the chronology of the Sonic games, Sonic has encountered a vastly-expanded cast of side characters, allies, rivals, and enemies, and has battled even greater threats to the world than Eggman himself in narratives that have become increasingly involved. Meanwhile, Sonic has starred in several animated TV series, and an expanded alternate universe for Sonic has been the focus of a long-running comic book continuity published by Archie Comics, which holds the Guinness world record for being the longest-running comic book based on a video game character.
It has been verified that nothing to do with Sonic or anything from his franchise exists in Melee at all, in spite of the infamous rumor detailed below. However when Yuji Naka was asked if Sonic had appeared in Melee in an interview with Edge Magazine, he stated that "It was very close, but time constraints did not allow us to continue with the idea." 
In the April 2002 edition of the video game magazine EGM (Electronic Gaming Monthly), an Aprils Fool's claim was that Sonic and Tails, the two biggest mascots of the game company Sega, could be unlocked as playable characters in the game Super Smash Bros. Melee by defeating 20 or more Fighting Wire Frames in Cruel Melee.
Players have proven this rumor false both in premise and in practice. It would be highly unlikely that Sega (which, during Melee's development, was not yet a full-fledged third party company, and thus was in competition with Nintendo) would sell its characters for use in a Nintendo game. There are no provisions to include Sonic and Tails in Melee's All-Star Mode (which showcases every playable character in the game), and an in-game message also indicates that Mr. Game & Watch is the last unlockable character (or whoever the player unlocks last). In addition, another message tells the player that they have unlocked every trophy. As beating single-player modes with Sonic and Tails would yield new trophies, this is impossible. Along with all of this, hacking the game reveals absolutely no data of Sonic or Tails existing in the game.
Additionally, Cruel Melee strategies showed very quickly that Sonic and Tails did not appear after obtaining 20 kills - in particular, a video of a Japanese player KO'ing 565 Wire Frames with Pikachu, along with an older video of a Danish player getting 10,000 kills with Samus disproved the rumor spectacularly. They can be seen here and here.
The Sonic franchise makes a sensational debut in the Smash Bros. series in Super Smash Bros. Brawl.
On the final character select screen (after all characters are unlocked), Sonic occupies the ninth column (miscellaneous characters) along with Mr. Game & Watch, Snake, and the random option.
 Assist Trophy
Note-With the exception of the Classic Sonic stickers, Sharhra the Genie and Erazor Djiin stickers, all the sticker art is character artwork that originates from Sonic Channel, the official Japanese website for the Sonic franchise.
 In Super Smash Bros. 4
The Sonic series returns for Super Smash Bros. 4, bringing the same character and a new stage.
 Games with elements from or in Super Smash Bros.
 Sonic the Hedgehog
Green Hill Zone and its respective music appears, as well as Sonic himself.
 Sonic the Hedgehog 2
Tails, who originated in this title, can be seen in the background of the stage, Green Hill Zone. In addition, the music for the Emerald Hill Zone can be heard, unaltered, in Super Smash Bros. Brawl. Plus, Super Sonic, Sonic's Final Smash, made its first appearance here.
 Sonic the Fighters
Many elements from this game are used for Sonic in Super Smash Bros Brawl.
 Sonic: Lost World
The first level in the game Windy Hill Zone appears as a stage for Super Smash Bros. Wii U.
 Other games
Sonic and other characters use artwork from newer Sonic games, like Sonic Adventure, Sonic Advance, and Sonic Rush. Respective stickers and trophies also appear. The SSBB version of "Angel Island Zone" was included in a compilation album titled True Blue: The Best of Sonic the Hedgehog. This version also appears in Mario & Sonic at the Winter Olympic Games and Mario & Sonic at the London 2012 Olympic Games. In addition, a statue of Sonic in the former game uses Sonic's pose from Brawl.