Shadfinite

Shadfinite is the slash ship between Shadow the Hedgehog and Infinite from the Sonic the Hedgehog fandom.

Canon
Shadow and Infinite first met while the former was storming one of Eggman's bases in the jungle. During the assault, Shadow wiped out the members of Jackal Squad that Eggman had hired for defense. Infinite - the leader of the squad - managed to catch up with Shadow and vowed to defeat him as vengeance for his team and to prove he's the "Ultimate Mercenary". However, Shadow showed why he's the Ultimate Lifeform and effortlessly beat Infinite. As Infinite laid defeated, Shadow told him he was worthless and to never show his "pathetic" face again before continuing his assault. Now alone, Infinite sees he's shaking from the encounter and realizes that he's experiencing fear, which he compounds with Shadow's words into believing he called him weak. This infuriates Infinite as he screams that he's not weak, and he attempts to prove that by abandoning his old self (signified by hiding his face behind a mask) and undergoing a process that merged Eggman's Phantom Ruby prototype into his chest.

A few months later, Shadow - now assisted by Team Dark - infiltrates another Eggman base, which turns into an assault thanks to Omega. Realizing his base is under attack by Omega, Eggman sends Infinite out to deal with him, which the jackal easily does. With contact lost, Shadow heads into the base to find Omega, only to find the reborn Infinite eager for another chance at the hedgehog. Shadow initially doesn't recognize Infinite, which the jackal reveals he suspected before recounting their previous encounter. As he finishes, Infinite points out to Shadow that the encounter made him realize that he had to cast aside his old self for being so weak and now Shadow's nothing more than an insect to crush. As a demonstration, Infinite uses his Phantom Ruby to trap Shadow in a virtual reality space, although Shadow manages to break out of it. While surprised at that, Infinite's happy with the result and says that not even Sonic can beat him now before flying off, leaving Shadow to ponder what has just happened with Rouge over the radio.

The two don't have any more personal encounters after that, but they still work against each other during the ensuing war after Infinite does beat Sonic. Shadow is one of the individuals that Infinite makes Phantom copies of to help in that fight, and ultimately to help Eggman's warpath. Shadow, himself, didn't join the Resistance but still worked against Infinite and Eggman in the 6 months Sonic was captured. At some point, he learned about Infinite's Phantom Ruby copies and their properties, which he shared with Sonic after his escape. Shadow then fought with the Resistance in the final battle against Infinite's army. He smiles with satisfaction when the Rookie thwarts Infinite's final attack of an artificial sun.

Fanon
Shadfinite isn't a popular ship for the Sonic fandom. Shadow being the cause of so much personal loss for Infinite, and the jackal's hatred of him because of that, makes it hard for fans to see how a relationship could start and dvelope. That said, it does have a significant following for those that do support it. It's rivaled by more popular ships for the both of them like Sonadow and Rookinite.

On AO3, this ship is decently written about with tagged entries in the 60's range. On Shadow's side, this ship is beaten out by other popular ships like Sonadow and Shadamy. For Infinite, this ship is his second-most written ship behind Rookinite.

Fandom
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Trivia

 * Shadow's theme in Sonic Adventure 2, "Throw it All Away", is briefly used in the track "Infinite's Beginning."
 * Shadow and Infinite's history is similar to him and Mephiles in Sonic 06, as Shadow plays an important part in their backstories yet doesn't remember them. In Sonic 06, Shadow time-travels to the past and seals Mephiles. When Mephiles returns, he is amused at how Shadow is the one who released him, though Shadow, who had not sealed Mephiles yet, doesn't know who he is.
 * Sonic Channel's January 2023 story features Shadow and Infinite in an alternate universe scenario where they're both singers.