Rival bosses

Rival Bosses are a recurring theme throughout the Sonic franchise. They are different from a traditional boss battle in the sense that they usually differ drastically from traditional boss encounters, or are outright stated as something different from the normal boss battle. Rival battles first appeared in Sonic Adventure, where they were quick 3 hit encounters with the other player characters. They have changed substantially within each installment but, are almost always easy to spot when compared to their counterparts, traditional boss battles.

In Sonic Adventure
In Sonic Adventure (and by extension it's remake Sonic Adventure DX: Director's Cut) rival encounters are short 3 hit boss encounters between certain characters in specific situations, during the story mode. These events are unique in that they do not appear in boss mode, and change depending on which character's story the player is going through at that time, meaning each view point of the encounter is viewed throughout the story mode. The boss encounters are as follows:
 * Sonic the Hedgehog (Fights Knuckles, Gamma)
 * Knuckles the Echidna (Fights Sonic)
 * E-102 Gamma (Fights Sonic)
 * Miles "Tails" Prower (Fights Knuckles, Gamma)

Strategy
The strategy for each of these is simple. Deal three hits of damage to win. Each of the bosses are allowed the abilities that they typically use when you play as them in their respective stories. They generally reserve one of their abilities for when they have been hit twice (e.g., Knuckles doesn't glide until then). But, they are fairly easy and usually will only last a couple seconds. They also have the ability to clash with your attacks, flinging both participants back a ways in a daze before resuming battle. Generally strategy is not essential here.

In Sonic Adventure 2
In Sonic Adventure 2, the rival boss encounters were given an overall makeover to be comparable to traditional boss battles. Due to the change in gameplay focus, relying on three different styles split among the cast members instead of Adventure's unique style per character; each character has a specific rival based on gameplay style (e.g., Sonic fights Shadow, Tails fights Doctor Eggman, etc.). Each rival is fought twice during the stories of the game, with the exception of Knuckles and Rouge, who only fight once in their respective stories.

Rivals

 * Sonic the Hedgehog and Shadow the Hedgehog (Fought after (In order): Metal Harbor, Final Rush. (Sonic) White Jungle, Final Chase (Shadow)
 * Miles "Tails" Prower and Doctor Eggman (Fought after (in order) Nothing, (First mission/boss) Crazy Gadget, (Tails) Weapons Bed, Cosmic Wall (Eggman)
 * Knuckles the Echidna and Rouge the Bat (Fought after (In order): Meteor Herd (Knuckles) Mad Space (Rouge)

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Main Articles:Sonic the Hedgehog (Sonic Adventure 2),Miles "Tails" Prower (Sonic Adventure 2),Knuckles the Echidna (Sonic Adventure 2),Shadow the Hedgehog (Sonic Adventure 2),Doctor Eggman (Sonic Adventure 2),Rouge the Bat (Sonic Adventure 2) ===== Unlike in Sonic Adventure, the boss encounters use a more traditional health bar system for their battles. Also notable, is that each boss uses their signature moves from the multi-player battle mode in their second encounter.

In Sonic Heroes
In Sonic Heroes, the battles (now called "Team battles") are changed yet again to favor a change in mechanics. In this game, the character stories are changed to feature three members per story; a speed, a power, and a flight type. Each member is active throughout the entirety of gameplay, which allows you to switch characters at will, (as opposed to one character being used per level with a different gameplay style, as per the previous installments). Because of this, the team battles are focused on a 3-on-3 ring-out style of battle.

Team Encounters

 * Team Sonic Vs. Team Rose (Power Plant area)
 * Team Dark Vs. Team Chaotix (Power Plant area)
 * Team Sonic Vs. Team Dark (Lost Jungle area)
 * Team Rose Vs. Team Chaotix (Lost Jungle area)

Main Articles: Team Sonic (boss), Team Rose (boss), Team Dark (boss), Team Chaotix (boss)
Your goal is to knock out each member of the opposing team until none remain. Knocking one member out of the ring disables the opponent from switching into that character, effectively disabling that type for the rest of that battle. Power-ups can be found around the arena, allowing you to increase your character's abilities. However, opponents can obtain these as well, so keep an eye on your surroundings. They have no real style of battle (besides the abilities granted by their current formation), so just keep attacking until they're all knocked out.

Console/PC
Unlike previous games, where the rival bosses contain some cohesive theme, they share barely any similarities in their appearance in Sonic Generations, aside from the fact that they all require you to run throughout the entire battle. This is unlike normal battles in which, there is usually some point where your momentum is stopped (whether it's for a moment, or throughout most of the battle). Each rival rests in the White Space world map and await a challenge from the player. It is mandatory to confront the rivals in order to obtain the Chaos Emeralds, along with each normal boss.

Rival bosses

 * Metal Sonic (Located in Chemical Plant area)
 * Shadow the Hedgehog (Located in City Escape area)
 * Silver the Hedgehog (Located in Crisis City area)

3DS
In the 3DS version, each rival boss is presented as a race through a previously cleared level. Also, due to the appearance of Special Stages, they no longer present you with Chaos Emeralds upon defeat. However, defeating them is mandatory in order to progress throughout the game.

Battle Locations
Metal Sonic- Casino Night

Shadow the Hedgehog- Radical Highway

Silver the Hedgehog- Tropical Resort