It sounds like you might be thinking of Ally of Justice Catastor, which is a card from the game.
To answer your question about its rarity: Ally of Justice Catastor is a Super Rare card in its original set, "The Shining Darkness."
Rarity levels in Yu-Gi-Oh! generally indicate:
* Common: Most frequent, usually found in large quantities.
* Rare: Less common than Common, but still relatively easy to find.
* Super Rare: Less common than Rare, usually found in smaller quantities.
* Ultra Rare: Least common, found in the smallest quantities.
This means that while Ally of Justice Catastor is not the rarest card, it's still considered less common than a normal Rare card.
If you're looking to find a copy of Ally of Justice Catastor, you'll likely need to search online or at a local card shop.