Roku was the avatar before Aang in Avatar: The Last Airbender. Thus he’s actually Aang in a past life, sorta. Reincarnations are a little strange an Avatar.
He was once the best friend of Fire Lord Sozin. But after he was revealed as the avatar, their friendship started to crumble over the decades. Roku was very angry at his former best friend for conquering land belonging to the Earth Kingdom.
Roku had to deal with an erupting volcano and at first thinks Sozin came to help.
But in truth, when he found Roku in a state near death, he let his former best friend die so he would not stop him in his global conquest. Really sad to be honest.
While Aang and Roku are technically the same soul, his personality remains in tact to teach Aang how to save the world. I really wish we saw more of him. I was shocked to learn he was the maternal ancestor of Zuko and Azula. Talk about irony.