Short comics released on the official Japanese Sonic the Hedgehog website, Sonic Channel, as part of the franchise's 25th anniversary celebrations. Each comic focuses on individual Sonic characters featured in the Character Profiles on Sonic Channel often featuring slice-of-life moments or references to their backstory in past games. (Source: Sonic Wikia, edited)

Sonic the Hedgehog Story Comic is a three-volume promotional comic for Sonic the Hedgehog distributed alongside the Japanese magazine Mega Drive Fan. 
The comic tells the story of the first game, with Sonic traversing across South Island and rescuing his animal friends from Dr. Eggman. 
 
As the comic was created before the final release of the game, there is a number of references to old concepts and prototype material. Sonic is the lead singer of a band, travels through Sparkling Zone and Clockwork Zone, and the final issue details how Sonic became the mascot of a deceased pilot.

Manga series focused on Shadow The Hedgehog in order to promote Sonic X Shadow Generations. 
Shadow and Maria live on the Space Colony ARK, but one day they are suddenly attacked by the Earth Defense Forces who are “disposing of dangerous biological experiments”. Shadow and Maria try to protect each other, but Maria is hit by a merciless bullet…? 
(Source: Shogakukan)

A manga booklet given out to Japanese movie attendees for Sonic the Hedgehog 3.

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