Guest is fire lord Ozai's daughter and a daughter of a water bender making her the sister of Zuko. Ozai had an affair with the water bender cheating on Zuko's mom. Obviously Zuko never took that anger out on Guest but one day when Guest was 6 years old she was banished to the earth kingdom because she turned out to be a water bender and not a fire bender. Zuko had many sleepless nights because of it especially since Guestwas the only one who put azula (the other sister) in her place