Yun is a card that corrupts beings. Formally known as the monstrous character now known as the hideous character.
AveEditOn Ave it was found on his forehead. On Ave it focused mainly on changing his body. He was mainly himself up until episode 20. He broke free with the help of Luva(Rai's Dad) and his heart/caring card. Although freed he is now a child but that was due to another character.
On Jahara it was found on her neck. It focused mainly on changing her mind. It turned her personality more then it did Ave. Under the cards influence Jahara attacked her allies more easlily then Ave did. She was freed from the effect with the help of Donha who made her fight the card and with Garnia's help by allowing the force onto himself. She was affected by Yun again after.
The symbol looks like the kanji 凶 (wicked)