Knuxmerald

MasterKnux is the cargoship between Knuckles the Echidna and the Master Emerald from the Sonic the Hedgehog fandom.

Canon
In most canons, Knuckles is the guardian of the Master Emerald.

Sonic &amp; Knuckles
On Angel Island, an Eggrobo stole the Master Emerald, prompting Knuckles to chase after the Badnik. The echidna was able to destroy it by luring Mecha Sonic as the latter tried to use a Spin Attack on him. Despite Mecha Sonic using the Emerald to increase its power, Knuckles defeated it nonetheless and was able to return the Emerald back to the island with the help of Sonic and his Tornado.

Sonic 3 &amp; Knuckles
When playing as Sonic and/or Tails, he/they will later in the game reach the Hidden Palace Zone, fighting Knuckles along the way. After defeating the echidna once, Sonic and/or Tails will end up in a room where the seven Super Emeralds and the Master Emerald lay standing. Suddenly, Dr. Robotnik in his Egg Mobile snatches the Master Emerald out of its shrine and got away, not before shocking Knuckles as the echidna clung up on it. The trio activated a teleporter to Sky Sanctuary Zone. Eventually, Sonic or Tails defeats Dr. Robotnik and both of them return the Master Emerald to Angel Island. Knuckles' storyline takes place after the aformentioned events, but the gameplay and storyline are almost the same as in Sonic & Knuckles. In the latter half of the game, the Master Emerald transforms the seven Chaos Emeralds into Super Emeralds. When all seven Super Emeralds are found, Knuckles can be transformed into Hyper Knuckles.

Sonic Underground
While the Master Emerald itself is not mentioned by name, the Floating Island's Chaos Emerald serves the same purpose. Knuckles' first priority was to stay and guard the Chaos Emerald in order to keep the island from falling, despite protests from Sonic and his siblings. When the Emerald got stolen, Sonic helped Knuckles get it back before the island hit the ground.

Sonic X
The Master Emerald remained unguarded when Knuckles was transported away from Angel Island. Knuckles made it his mission to return home in order to reunite himself with the Emerald.

The Chaos Emerald arc follows the same storyline as in the Sonic Adventure game: Dr. Eggman summons Chaos inside the Emerald, shattering it out of the island, and Knuckles collecting the shards to return the Emerald to its full capacity once again.

In "Cosmic Crisis", Chris used the Master Emerald as an alternative to the already-scattered seven Chaos Emeralds to power up the Blue Typhoon. Provoked by the theft of the Emerald, Knuckles confronted Chris and demands to return it to the island. He was forced to accept the decision of letting it stay on the Blue Typhoon after Amy, Cream, and Cosmo "knocked some reason" on him. Later, the Emerald was used to power up the Sonic Power Cannon to destroy Dark Oak, draining it much of its power and once again shattering it in the process. According to Dr. Eggman, it would take years for the Emerald to return to its former powered-up state.

Sonic Mania Adventures
In "&amp; Knuckles", Knuckles was first seen carrying the Master Emerald in a forest within Angel Island. He then stopped to take a rest. Hearing a crash nearby, he thought of Dr. Eggman, the Heavy King, or Sonic attempting to steal the Emerald once again. Ray the Flying Squirrel landed beside the echidna and rolled around the Emerald. He greeted Knuckles and asks him where Mighty the Armadillo was. Knuckles shrugged, while Ray glanced and tried to touch the Emerald. Knuckles immediately held him and flung him to the sky. Later, Knuckles fought a trio of RhinoBots while Eggman stole the Emerald using the Tornado. Thinking Sonic was responsible for the theft, he chased after the biplane in anger.

In "Metal Mayhem", Knuckles charged through the lair and punched the already powering-up Metal Sonic, pushing both it and Eggman outside. The echidna then carried the Master Emerald out of the lair, unaware of the presence of Sonic, Tails, Ray, and Mighty nearby.

Fanon
Some fans tend to pair Knuckles with the Master Emerald instead of another character. With the release of Sonic Mania Adventures, artwork involving Knuckles romantically involved with the Emerald had some popularity.

Fandom
DEVIANTART
 * Knuckles and Master Emerald on DeviantArt
 * KnucklesXMasterEmerald on DeviantArt