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A malicious and dangerous Pokémon, it seeks revenge against the trainer who abandoned it. Its shackles were reinforced in order to keep its violent nature under control.
Chainite's dark spirit has become so powerful and angry that its Linkite body can no longer contain it and is literally bursting at the seams with malicious spirit energy. Wisps and tendrils of black smoke emerge from the Substitute doll's arms, feet, and mouth. Chainite's shackles are much heavier than Linkite's and it has a straitjacket and a large padlock in front of it. The Poké Ball at the end of its chain has grown spikes and can be used as a flail.
Linkite is the only Pokémon that evolves by trade that gains a type immunity from doing so.
Chainite is based on a Substitute decoy chained to a Poké Ball, presumably the one it died in. Its bonds have been reinforced with a collar, padlock, and straitjacket.
The shiny, as EeveeBaily put it, "it's supposed to represent a Love Ball, which makes it even more tragic that it got abandoned by its trainer." However, the irony from a gameplay mechanic standpoint is that Love Balls don't have any effect on the friendship stat like the Friend Balls or Luxury Balls do. Instead, Love Balls only increase the odds of catching Pokemon of the opposite gender, making the ties to the Love Ball even more ironic given that Linkite and Chainite are Genderless, and thus would be unaffected by it.