Cloud AU's

Time tells

Skull has memory loss als the others have to tell him about himself and their adventures.

Or:
Skull loved to sing as the others soon find out with him suddenly humming all the time.
He eventually sang 'Colours of the Rainbow' and told them it was an anniversary gift to their first meeting and becoming Arcobalenos.
Weirdly enough, the others were sure they had heard the song in their childhood before.
By a... traveling performer that ran away from home or smth.
It was during their darkest hours they'd met or seen the guy,
as for how he was dressed...
they can't recall.
They'd also heard it in a dream before they became Arcobaleno as well as the Representative Battle.

Eventually Uni consulted Kawahira along with Skull.
Kawahira looked between Skull and her multiple times.
Then proceeded to make Skull sing that Song again?
...Kawahira practically sank in his seat while listening to it, garnering concerned or questioning looks.
At the end he told them Skull was/ is their Generation's... 'Heart' of sorts. Like, the one they always protect or keep a tight connection with.
Who they can go to for their problems... like a Sky, which they usually are, but it seems that instead of Aria or Uni that role had went to Skull instead as he claims people and thus keeps their secrets safer, not betraying them.

...Clouds call the Storm, Rain and Lightning into Action to protect the Sky even if their far away,
they cover up the Sun when things get to rough, Mist can't exist if the Clouds aren't there to bring Rain and Cover the Sky from seeing it's path on the Ground.

And let's be honest,
despite them thinking he's annoying, loud, weak and useless, they still haven't even thought about kicking him out of the Arcobaleno. They talk to him,
treat him like one of them, try to give him some training to protect himself and care for him in their own way. That's because he's their Heart that still posesses the humane traits they abandoned or shut out.