The pairing only appears in the Sonic Underground cartoon series.
In "Friend or Foe?", Sonic, Sonia and Manic, in their search for their mother, traveled to the Floating Island, being told by the oracle to "beware the echidna". Sleet and Dingo were the first to meet Knuckles on the island after being captured by the echidna. The duo convinced Knuckles that they were here to stop the "real invaders", using a holo-projector to fabricate the three hedgehogs planning an invasion. Knuckles releases them and were told to leave the island. Meanwhile, the hedgehogs reach a sacred pool and unknowingly activated it. Knuckles confronts the trio and called them hedgehog thieves. Sonia asks Knuckles if there are any good dry cleaners on the island. After a short fight with Sonic, Knuckles activated pit traps prepared for the hedgehogs, in which Sonic escaped easily and allowed himself to be chased by Knuckles. Sonia and Manic explored the underground of the Floating Island and befriended Knuckles's pet dinosaur, Chomps. They were able to gain valuable information from the dinosaur regarding the holo-projector he coughed up, and Sleet and Dingo being on the island. When the hedgehogs were fighting the fleeing duo with the Chaos Emerald, Knuckles was tricked into thinking that they were his enemies, although he eventually found out the truth. With the help of the hedgehogs, Knuckles was able to drive Sleet and Dingo away while Sonic recovered the Chaos Emerald. Later that night, Knuckles treated the hedgehogs to krupnik and asked their purpose of coming to the island. When Manic replied that they were searching for their mother, Knuckles told them the fact that he has met their mother when he was young.
In "Flying Fortress", Knuckles spotted an unidentified aircraft using radar on the Floating Island. Convinced that the aircraft was a "Robotnik-built flying Swatbot", he commands the island's anti-aircraft system to fire it at will. With the Swatbot heavily damaged, Sonic maneuvers it into inscribing his name using smoke on the sky to signal Knuckles to cease fire. The hedgehogs land the Swatbot on the island, and informs Knuckles of a Chaos Emerald-powered flying fortress. Baffled by the existence of other Chaos Emeralds, Knuckles asserts his first duty of protecting the Floating Island. Sonia tells him that he is part of Mobius and if it goes, so goes the island. The hedgehogs sing to Knuckles about no one being an island, and he accepted their plea for help; after which they were able to infiltrate into the fortress.
In "No Hedgehog is an Island", Sleet and Dingo took the fortress' Chaos Emerald for themselves. Without a power source, the fortress fell down to the sea. As the fortress sank, Sonia found an means of escape through the garbage disposal, with Knuckles fearing to be "diced up like a plate of leftovers". Sonia replies that Robotnik would only dump his garbage and pushes Knuckles into the chute, which actually leads them outside. Knuckles uses a rope to pull the hedgehogs to an island. Sonia tells Knuckles how peaceful the island is and to forget about everything. Knuckles replies that he cannot forget about the Chaos Emerald and should have stayed on the Floating Island. As Chaos Emerald energy hit the beach, creating an earthquake, Sonia is dragged down. Knuckles digs a hole down to the cracks and was able to bring Sonia back up ground. He confirms with Sonia that the released Chaos Emerald energy without controls is chaos all over Mobius. Later, Knuckles joins the hedgehogs in the search for the Chaos Emerald. Sleet confronted the group but failed to catch them. As the hedgehogs fled from Sleet, Sonia asked where Knuckles was, with Sonic thinking he was behind him. Meanwhile, Knuckles hid in a crater without joining them and requested Sleet to bring him to Dr. Robotnik.
In "New Echidna in Town", Knuckles makes a deal with Dr. Robotnik in order to save Mobius, even though Sonia and her brothers have told him not to. Knuckles began to regret his actions of betrayal towards Sonia and her family when Dr. Robotnik was getting ready to roboticize her first, but Knuckles steps in to save Sonia and helps the hedgehogs to escape before they face a larger challenge. After a long battle with Dr. Robotnik over a Chaos Emerald, Knuckles allows the hedgehogs to stay on the Floating Island if they ever need a break. Sonia replies that Knuckles is "welcome to sit in the band anytime" and kissed him on the cheek. The episode ends with them gazing on the Floating Island.
On AO3, the pairing is the third most written ship for Sonia, behind Bartonia and Sonic Underground. However, ships involving Sonia are rare, as it is uncommon on the website.
- knuxonia tag on DeviantArt
- knucklesxsonia tag on DeviantArt
- soniaxknuckles tag on DeviantArt
- ForAllKnuxoniaLovers fanclub on DeviantArt
- Knuxonia-Manally fanclub on DeviantArt