In the second book, The Shadow of Kyoshi, we learn that Avatar Kuruk actually had a pretty tragic life.
A big plot point of The Shadow of Kyoshi is that Avatar Kuruk is trying to get in contact with Kyoshi, but she’s having a hard time reaching him because she can’t stand that version of her past life since she (much like the Avatar fan base) deem him to be such a failure.
I won’t spoil why, but I’m glad that his character gets retconned in The Shadow of Kyoshi.
But through their encounter, we learn that Avatar Kuruk is one of the greatest Avatars ever and that he actually saved the mortal world.