What it does
Contrary reverses all stat changes. Moves that lower a stat raise it instead, and moves that raise a stat lower it instead. This applies to the holder's own moves and to any stat drops the opponent tries to inflict.
The reason Serperior runs this at 96%: Leaf Storm has a built-in penalty that drops the user's Sp.Atk by two stages. Contrary flips that penalty into a boost — every Leaf Storm raises Sp.Atk by two stages (×1.5). After the first use, Serperior sits at +2. After the second, it's at +4 (×2.0). That's not setup — that's just what clicking the attack does. It snowballs by design.
Side note: Intimidate normally drops Attack, so under Contrary it would raise Serperior's Attack instead. Serperior runs special attacks, so this rarely matters in practice. What does matter is if the opponent tries to use something like Fake Tears to drop Serperior's Sp.Def — that backfires immediately.
Who runs it
Serperior at 96%. That number means: any Serperior you see in Champions almost certainly has Contrary. This ability is its identity — without it, Serperior has a modest 75 base Sp.Atk and isn't particularly threatening.
How to use it
The loop is: switch in, use Leaf Storm, get +2 Sp.Atk. Use it again, get +4. At +4, Serperior is dealing double its base output from Leaf Storm alone. Substitute is a common addition to absorb one hit while you build, giving yourself time to reach the second or third Leaf Storm. Dragon Pulse or Glare give coverage or speed control when you don't need to press Leaf Storm that turn.
Tips & strategy
The three realistic answers to a Contrary Serperior: KO it before it gets a second Leaf Storm off, bring something faster that outspeeds it, or use Trick to force a choice item onto it — locking Serperior into one move takes away most of its flexibility.
Serperior's soft points are its physical bulk and the fact that 113 Speed, while fast, isn't untouchable. Priority moves and the fastest tier of attackers can deny the snowball before it starts. Build your team to give it clean switch-in opportunities, and its job is to run the loop.