Yeiju was a little street urchin that was taken in by a blacksmith. That blacksmith is called Shin Yu, and started teaching Yeiju his craft in return for food and shelter.
Shin Yu became something of Yeiju's adoptive father. He's the person Yeiju looks up to, and the one he thinks of when he tries to imagine the kind of person he wants to be someday. Shin Yu's imagined to be a stubborn, strong-willed person, with a heart of gold but a rather gruff exterior and a take-no-shit attitude. He helps others, but he isn't at all a pushover by any means. One has to be a pretty tough guy to be able to handle working with fire and metal as much as he does. He is a firebender, as well, and started teaching Yeiju advanced firebending techniques later on.
Shin Yu also took in Rys without hesitation, and protects her from her abusive ex-boyfriend. Currently they're in the Fire Nation, though they work out of Ba Sing Se's Middle Ring, in order to reuse war weapons in non-war-related efforts.