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A Hidden passive is a little-known feature in League of Legends that has very little effect on actual gameplay and exists for simple amusement. There are only 3 known gameplay altering hidden passives in League of Legends:
Law of Inverse Ninja Strength: Ninjas are more effective when they work alone. -1 HP for each other allied ninja. This applies only to the ninja characters: Akali, Kennen, and Shen.
Prowl: Nidalee's innate ability Prowl grants her the power of a "cougar". Nearby allied champions gain +5 experience if they are a lower level than Nidalee upon her level up. 
Leona's passive Sunlight deals one less damage to champion skins wearing sunglasses (eg. Surfer Singed and the Commando skins).
When the first Champion Spotlight was released for Lee Sin, it was stated that his hidden passive was blindness, which made the entire screen blurry while playing him at all times, but rendered him immune to abilities such as Teemo's Blinding Dart and Nocturne's Paranoia. However, this turned out to be an April Fool's Day joke
It has been confirmed that other characters who have labeled icons on their character screens ( Gangplank's pirate icon and Miss Fortune's pirate hunter icon) do not have code that alters gameplay.
Not sure if these are true or not but they could be easily tested.. Just interesting lol.