Keller, Tae-When You Trap a Tiger

Would you make a deal with a tiger? 

Lily has a superpower--she can turn invisible. Well, not really, but being shy and quiet means that Lily hears and sees things she shouldn't. Like her mom and older sister fighting.. or the magical tiger straight out of her halmoni's Korean folktales. Long ago, Lily's grandmother stole something from the tigers. And now they want it back. The deal seems almost too good to be true: return what her grandmother stole in exchange for Halmoni's health. 
With the help of her sister and her new friend Ricky, Lily is determined to save her grandmother. She just needs to find her voice and a new superpower: the courage to face a tiger.