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.
As Sega's 16-bit console, the Genesis, was launched by 1990, Sega decided it needed to develop a killer-app for the system that could effectively combat the 1991 launch of the Super Nintendo console and its hit pack-in game, Super Mario World. It put into effect "Project Million Seller", where a small team of developers, prominent among them programmer Yuji Naka, developed a side-scrolling platforming game named Sonic the Hedgehog. The game indeed sold millions as the pack-in title for Genesis systems, thanks in no small part to design elements that were welcomely radical and fresh for their time: lengthy-yet-timed action-based levels where the object is to get to the end before time runs out, and the main character runs and jumps very fast through these levels to bounce off of and roll through enemies and obstacles while avoiding pits and spikes and bouncing off springs. This character, the anthropomorphic blue hedgehog and 'tude-filled speed demon named Sonic, was endearingly established as Sega's mascot and answer to Nintendo's Mario and helped ensure the Genesis' place as the primary contender to the SNES in what would be remembered as one of gaming's most memorable "console wars".
As one would expect for such a smash hit character and game franchise, sequels were in order, and a steady stream of Sonic sequels, spin-offs, and alternative media was released in all of the following years for all Sega-owned systems. The "real" sequels to Sonic the Hedgehog were generally the most successful for expanding on both the core gameplay and the characterization and world design. Sonic the Hedgehog 2 introduced many Sonic staples, such as Sonic's Spin Dash ability to charge forward at full-throttle, Sonic's Super Transformation ability to become a faster and strong gold-colored version called Super Sonic, and a CPU-controlled sidekick to follow and assist Sonic, which in this case is the young two-tailed fox Miles "Tails" Prower. Sonic the Hedgehog 3 thoroughly revamped the gameplay and presentation and introduced Sonic's hero rival Knuckles the Echidna, and Sonic & Knuckles was a direct and physical continuation that was the first and only Genesis cartridge to have another Genesis cartridge insertable onto it. Locking Sonic 3 into Sonic & Knuckles created a huge Sonic quest where Sonic, Tails, and Knuckles were all separate playable characters.
Time would not be so merciful to Sonic's career and Sega's standing in the game industry in the years afterward, though. A lot of spin off games for Genesis and other less successful Sega systems like the Sega Saturn took the franchise in various different directions such as isometric platforming and adventure-based racing, and also introduced more characters, but the games were not appreciated as much as the main series by fans and critics alike. Sonic had a magnificent resurgence on Sega's better-handled Dreamcast console with its premiere title Sonic Adventure, successfully pulling off a fully 3D adventure game that physically remodeled Sonic's character design and yet felt worthy of the Sonic name. In spite of great hardware and software in the Dreamcast, the system and Sega itself were ultimately defeated by the competition in the form of the powerful Sony PlayStation brand (with franchises like Metal Gear Solid) and the phenomenal popularity of Nintendo's Pokémon franchise, and Sega ultimately shifted its company focus to strictly a software publisher.
Now a developer for companies once considered its rivals, Sega resumed developing a steady stream of Sonic titles and releasing them for all three main competing hardware publishers - Nintendo, Sony, and Microsoft, though it started exclusively for Nintendo with Sonic Adventure 2: Battle being the first Sonic game for the GameCube. Some of these games would be critically acclaimed, some viewed as mediocre, and some widely panned, and standout Sonic games on Nintendo systems include the Sonic Advance series for Game Boy Advance, Sonic Rush for DS, and Sonic and the Secret Rings for Wii. As a franchise, Sonic remains one of gaming's most recognizable, and for this reason Sonic the Hedgehog has even appeared in crossover games by popular demand. Three Wii titles are especially notable: one is Mario & Sonic at the Olympic Games, a track-and-field-style sports title featuring characters from the Mario universe and the Sonic universe competing against each other; its sequel, Mario & Sonic at the Olympic Winter Games; and Super Smash Bros. Brawl, featuring characters from many universes as playable fighters, with Sonic among those playable.
The Sonic game franchise is a widely ranging series of games and other media covering many genres and formats, and if the characters and world consistent among them were taken away, there would seem to be several games and sub-series unrelated to each other and to the "main" series of side-scrolling platformers left behind as a result. Sonic and related properties have appeared in games covering genres such as side-scrolling, 3D adventure, racing, party-gaming, pinball, sports, and fighting. What is generally consistent among all Sonic games is a sense of speed and fast pacing, and gameplay and action is generally emphasized over voluminous characterization and overwrought storytelling. A sense of "attitude" and "coolness" pervades each Sonic title as well.
The Sonic franchise comprises several separate continuities and alternative interpretations. In many cases, the games are not groupable together by "storyline". In general the franchise depicts a world where characters are wildly anthropomorphic and colorful animals who speak and interact just like normal people (similar to the Star Fox universe). "Normal humans" also exist in the Sonic world, however, and the primary antagonist of the series is a "mad scientist"-style human named Dr. Ivo "Eggman" Robotnik who repeatedly attempts to take over the world with an army of robots and a Death Star-inspired super-weapon named the Death Egg, and his nemesis Sonic must thwart his schemes time and time again, collecting power-ups like Gold Rings and powerful artifacts called the Chaos Emeralds along the way. Recent Sonic games have explored different and more involved styles of storytelling and added new characters of good and evil affinities, much like recent Mario games.
In Super Smash Bros. Melee
There is no verifiable mention of anything Sonic related 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." 
Sonic & Tails Rumor
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.
In Super Smash Bros. Brawl
The Sonic franchise makes a sensational debut in the Smash Bros. series in Super Smash Bros. Brawl.
- Sonic the Hedgehog: The eponymous star of the franchise was the most anticipated and requested third-party character for Brawl. Seemingly as expected, he is a character who emphasizes speed, and has been compared to Fox and Falco in Melee. His Final Smash is Super Sonic which utilizes the seven Chaos Emeralds to transform him and fly around the stage, ramming into opponents at high speeds.
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.
- Shadow the Hedgehog: Shadow uses Chaos Control on the battlefield, which causes the movement of opponents to slow down.
- Green Hill Zone : Based on the first level of the first game. The stage is based almost entirely around a huge crater like formation at the center. The bottom areas of the dip are destructible and break away in squares. A checkpoint Star Post appears at certain places in random intervals which can be struck to make it spin, damaging enemies who touch it, other than the one that attacked it, unless it's a team battle, then it applies for all teammates. Also, Tails, Silver, and Knuckles make a cameo in the background of this stage.
- Green Hill Zone - Original Sega Genesis version from the actual level. It is the theme for the Green Hill Zone stage.
- Angel Island Zone: The theme of Angel Island Zone from Sonic the Hedgehog 3, completely redone by Jun Senoue, the composer of the music for the Sonic Adventure titles. It is used on the Green Hill Zone stage. This is the only Sonic the Hedgehog remix made for Brawl.
- Scrap Brain Zone - Taken directly from the original Sonic the Hedgehog, this was the theme to the game's last area. It is used on the Green Hill Zone stage.
- Emerald Hill Zone - Original Sega Genesis version from the 1st level of Sonic the Hedgehog 2. It is used on the Green Hill Zone stage.
- Sonic Boom - The main theme of the American version of Sonic CD which played during the opening movie, this song is taken directly from said game. It is used on the Green Hill Zone stage.
- Super Sonic Racing - Background music and main theme taken directly from the lesser known Sonic racing game Sonic R. It is used on the Green Hill Zone stage.
- Open Your Heart - The main theme of Sonic Adventure, this is a looped version of the very same song from said game. It is used on the Green Hill Zone stage.
- Live and Learn - The popular main theme of Sonic Adventure 2, it was performed by Crush 40, the same people who did Open Your Heart. It is used on the Green Hill Zone stage.
- Sonic Heroes - Also performed by the same band as the above two songs, this was the main theme of the game of the same name. It is used on the Green Hill Zone stage.
- Right There, Ride On - Taken directly from the first Sonic DS title known as Sonic Rush, this was the background music to the Leaf Storm levels. It is used on the Green Hill Zone stage.
- HIS WORLD (Instrumental) - An instrumental version of Sonic's theme from the next generation Sonic the Hedgehog title for the Xbox 360 and PlayStation 3. It is used on the Green Hill Zone stage. It is unlocked by playing on the Green Hill Zone stage 10 times.
- Seven Rings in Hand - Taken directly from Sonic's first Wii title Sonic & The Secret Rings, this was the main theme to that game. It is used on the Green Hill Zone stage. This song also plays during Sonic's Classic Mode credits.
- Sonic's victory theme - The victory fanfare of Sonic is an orchestration of the "Level Clear" theme from various Sonic the Hedgehog titles, with it originating in Sonic the Hedgehog 3. This version is taken directly from the next generation Sonic the Hedgehog for the Xbox 360 and PlayStation 3, making it the only victory music directly imported from another game.
- Amy Rose
- Blaze the Cat
- Cream the Rabbit
- Dr. Eggman
- Jet the Hawk
- Knuckles the Echidna
- Shadow the Hedgehog
- Silver the Hedgehog
- Sonic the Hedgehog
- Super Sonic
- Miles "Tails" Prower
- Amy Rose
- Big the Cat
- Blaze the Cat
- Charmy Bee
- Classic Sonic (Sonic the Hedgehog JP Ver.)
- Classic Sonic (Sonic the Hedgehog US Ver.)
- Cream the Rabbit and Cheese the Chao
- Dr. Eggman
- Erazor Djiin
- Espio the Chameleon
- Gamma (E-102)
- Jet the Hawk
- Knuckles the Echidna
- Metal Sonic
- Omega (E-123)
- Rouge the Bat
- Shadow the Hedgehog
- Shahra the Genie
- Silver the Hedgehog
- Sonic the Hedgehog
- Storm the Albatross
- Super Sonic
- Tikal the Echidna
- Vector the Crocodile
- Wave the Swallow
Note: With the exception of the Classic Sonic, 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. for Wii U and 3DS
The Sonic series returns for Super Smash Bros. for Nintendo 3DS / Wii U.
- Sonic the Hedgehog: Sonic returns in Super Smash Bros. for Nintendo 3DS/Wii U, with his same special moves from Super Smash Bros. Brawl.
- Shadow the Hedgehog: Shadow returns as an assist trophy. Like in Super Smash Bros. Brawl, he uses Chaos Control to slow everyone down, except the one character who summoned him.
- Eggrobo: The pawns created by Dr. Eggman from the Sonic series are confirmed as new enemies from Smash Run.
Stages (Wii U)
- Windy Hill: The first level of Sonic Lost World, Windy Hill, appears as a stage for Super Smash Bros. for Nintendo 3DS/Wii U.
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's side Taunt "You're too slow!" and running animation camed from Sonic CD,when Sonic charges his "Strike Dash",a hability that Sonic charge his run and make it faster.
Sonic the Fighters
Many elements from this game are used for Sonic in Super Smash Bros Brawl:
- Sonic's A attacks are based on the "punch, punch, kick" move he used in this game.
- Sonic's Dash attack is based on a similar move from this game in which the player has to run and attack.
- All of Sonic's tilts are based on moves he used in this game.
- Sonic's Neutral, Forward, and Down air attacks are based on moves he used in this game.
- Sonic's Forward Smash is based on a move he used in this game.
- Sonic's Down throw is based on a move he used in this game.
- Sonic first wall-jumped in this game; thus, that ability is given to him in Brawl.
Sonic Lost World
Windy Hill is a stage representing the Sonic universe in Super Smash Bros. for Nintendo 3DS/Wii U.
Sonic and other characters use artwork from newer Sonic games, like Sonic Adventure, Sonic Advance, and Sonic Rush. Respective stickers and trophies also appear.