A couple of friends and I are confused about a few things so I have come here to the experts.

How do Friend abilities work?

Does the effect of the ability only work when it comes into play?
How do you distinguish between the abilities of Yuffie and Kairi?

When you play Yuffie you can randomly discard one of your opponents’ cards in his hand. Can that ability be repeated every turn?
This question also applies to Hercules, Pluto, Abu, Kairi, really anyone who has the "down arrow".

What makes ability repeatable each turn? When Herc is put in play with Jack on the board can I use "Hercs Power" on the next turn?

Thanks in advance...

Also, can a player use a magic card like fire/blizzard/thunder during a challenge against a Player/Light?

With the "Down-arrow" symbol, it means that the ability activates ONCE, at the moment it is played.

The only thing that can make an ability repeatable is if it has no "Down-arrow", and has a requirement to activate. I think.

I think he's asking if he can play another card with the same ability and perform that ability again. In which case, the answer is yes, with as many Hercs, Abus, and Yuffies you have in your hand, you can repeat the action. This also would apply to Tigger, if you were allowed to have more than one per deck.