Viral Chao is a group that appears in Sonic Chronicles: The Dark Brotherhood. They are a group of unique Chao intended to be released at official events in the style of event Pokémon of the Pokémon games. There are five Viral Chao: Baki, Farfinkle, Fimbley, Pooki, and Stugs. However, Stugs was the only Viral Chao ever officially distributed to the public, as due to poor attendance and sales, the other four chao events were cancelled. However, game-altering devices can be used to unlock the remaining Viral Chao. Codes, which can be found in the official Japanese website of Sonic Chronicles: The Dark Brotherhood, can be used in the Chao Garden to unlock all but Farfinkle. The cheat codes, however, will only work for the Japanese version of the game.
All Viral Chao have the same basic appearance; a black body, a rounded head and a black head-orb. Each Chao also has a colored strip on their heads covering their eyes, resembling an eye mask, making each Chao somewhat resemble a ninja. These "face masks" are colored differently depending on the Chao: Baki's is pink, Farfinkle's is orange, Fimbley's is blue, Pooki's is red and Stugs' is white.
- Baki: Bonded character making a successful counter move will automatically get free attacks against the enemy. Additionally, the character's Damage stat is increased by two.
- Farfinkle: Bonded character automatically inflicts a random status effect on enemy when making a normal attack. Additionally, the character's Damage stat is increased by two.
- Fimbley: Bonded character will automatically be missed by up to three basic attacks/rounds.
- Pooki: Bonded character's Armor stat is increased by six and spreads the remaining damage evenly among other team members.
- Stugs: Bonded character avoids all enemy attacks except when the character is the last surviving target.
- Farfinkle was not included in the list of unlockable Viral Chao in the Japanese release because it has a severe glitch causing 'KO' to be a status effect. If this happens, the affected enemy may try a POW move but cannot, forcing the player to shut their Nintendo DS down. This is the only Chao not released to the public.