does kayle and morgana have a hidden passive

Nope

Zyra and Sion get +5 gold for killing each other (Plants vs Zombies)
Nidalee gives lower level champs passive 1 exp per second. (Since they are younger and she is a cougar)
Leona does 1 less damage to any champ wearing sunglasses or have their eye blocked (lee sin). Also her spells make Zyra grow larger for a few seconds.
If Maokai is close to Zyra he gains 1 extra MS.
Fizz's ult can "eat" champions with small models when they die, leaving no corpse.

There are also many secret emots for certain situations and rival champions.

Vi and Caitlyn have some interactions too. If they're allied they're "on the case" and if they're opposing Vi can have a special taunt for Cait.

yeah but there are quite a few champ that have hidden emots. Like Volibear and Zilean. Skarner will also say a special emot if he is standing still in a brush for 12 seconds. Graves taunts Nocturn with his smokescreen, etc. There are quiet a few hidden emots.