The Sonic the Hedgehog franchise has an incredible cast of characters within the video games and spinoffs. Although a video game character may have appeared in the comics, it was less likely to see a character originating from the comics in one of Sonic's mainline games. With such an ensemble cast, it would be obvious that so many of them have interesting backstories. Because of this, it's easier to narrow down which Sonic character has the stronger backstory by only looking at the cast of video game characters and exclude the spinoffs/non-mainline canon.
Most of these characters have taken a back seat in terms of plot importance in recent entries. In fact, some characters have only appeared in mobile spinoff games since their debut game. With the newly-announced Sonic Frontiers, fans are hoping that characters outside Sonic, Tails, and Eggman will return to play bigger roles in the story.
5 Shahra
Sonic and the Secret Rings was released in 2007, and it's safe to say it has an odd reputation within the franchise. For one, it came out just after the infamous Sonic the Hedgehog (2006), and was one of the many games during the fan acclaimed "Dark Age" of the Sonic franchise. Despite this, it gained a "sequel" in Sonic and the Black Knight in 2009, creating the "Sonic Storybook Series." Sonic and the Secret Rings focused on the titular character and introduced Shahra, a genie of the ring. As Sonic journeys with Shahra to stop the Erazor Djinn, he slowly learns more about her.
The main antagonist (Erazor Djinn) of Secret Rings is different from Dr. Eggman or Black Doom. Sonic finds out that Shahra and the Erazor Djinn were close prior to the events of the game. What makes Shahra's backstory so interesting is how eerily realistic bits and pieces of her past relationship is with the Erazor Djinn. It's never outright stated that they were lovers, but Shahra's reluctant actions and words in the penultimate climax ("I just want to be back with him") when Erazor Djinn tries playing on her emotions says enough for gamers to piece together. Shahra and (to an extent) Erazor are one of the few in-game relationships that was emotionally abusive, something that wasn't exactly common at all in a Sonic game. Sonic and the Secret Rings handles its story and cast with mature themes, and Shahra's relationship with Erazor Djinn may hit home for those who've been trapped in unhealthy relationships.
4 Silver the Hedgehog
While he may not have been as well-received as Shadow, Silver has made his place within the Sonic community. This psychokinetic hedgehog debuted in Sonic the Hedgehog (2006), where he traveled to the past to save his future from Iblis, the Flames of Disaster. Silver's primary goal is to kill Sonic, who he dubs the "Iblis Trigger." Despite his determination, Silver's plagued with doubts throughout his journey.
If Silver appears in a Sonic game, it's most likely because his future is in danger. While not much is shown of Silver's world beyond a destroyed city or word-of-mouth from Silver himself, it's the concept of the future that makes him interesting. So much about Silver's origins are still a mystery, which adds to his appeal, but it would be nice to explore Silver's world in the same way Sonic had with Blaze's dimension.
3 Blaze the Cat
Since her first appearance in Sonic Rush, Blaze the Cat has been an incredibly popular character among the fanbase. She's currently the only female Sonic character in the game continuity that has a Super Form. Blaze is one of the few characters that has much of her backstory already revealed. She was ostracized from others because of her pyrokinetic abilities and because of her role as the Sol Emeralds' Guardian. This alone had fans wondering more about her world and wanting to explore it outside the Sonic Rush games.
With how much fans currently know about her, it's a wonder why Sonic Team hasn't given Blaze her own spinoff game. Because although Sonic Rush Adventure built on Blaze's world, there's still so much of the Sol Dimension that remains a mystery.
2 Tikal the Echidna
Chronologically, Tikal and the Knuckles Clan were the first instances of echidnas within the Sonic franchise. With how Tikal's story ended, it's no wonder she hasn't appeared in a Sonic game. Sonic Adventure (DX) is renowned for its cast of characters and story. Throughout the game, Sonic and his friends are visited by Tikal's spirit, who shows them glimpses of her past and how Chaos came to be.
Even though she wasn't a playable character, Tikal's backstory provides some lore to the Master Emerald. In fact, her prayer that was passed down from her grandmother is used by Knuckles and comic-exclusive characters. Currently, Tikal and her clan remain the closest thing to Knuckles' past. Despite her one-time appearance as a plot-important character, Tikal has a decent fanbase within the community.
1 Shadow the Hedgehog
Shadow spent several games struggling with his past thanks to his amnesia, but it's Sonic Adventure 2 and Shadow the Hedgehog that really sinks their teeth into the lore behind the whole "Project Shadow." 50 years ago, Shadow was created to be the Ultimate Lifeform after the first prototype, the Biolizard, failed to meet standards. Following his creation, Shadow befriended Dr. Gerald Robotnik's granddaughter, Maria, who suffered from a terminal illness. Shadow and Maria's relationship was like brother and sister, and it was Maria's death that drove Shadow forward in the events of Sonic Adventure 2. Thanks to Dr. Gerald, there were holes in Shadow's memory, and he initially believed Maria wanted Shadow to enact revenge on humans for raiding and killing everyone on board the ARK.
Shadow's backstory is explored further in Shadow the Hedgehog, where it's revealed he's related to Black Doom, the leader of the Black Arms. Dr. Gerald struck a deal with Black Doom in exchange for Doom assisting with Shadow's creation. With both Sonic Adventure 2 and Shadow the Hedgehog, Shadow currently stands as one of the few main characters in the Sonic franchise with most of his backstory already uncovered.