Ying left her hometown with her husband to begin the trip to Ba Sing Se where they planned to start a safer life for their child that Ying was pregnant with at the time. When things got bad, Ying held onto hope that she and her husband would make it and, with a little help from the Avatar, they did.
Aang and his friends are about to leave a waterfall when they run into Ying and her family. Katara invites them to travel with them, but the family tells them that the Serpent's Pass is far too dangerous. Instead, they plan on traveling via an underground ferry system that leads refugees to Ba Sing Se. Aang and the others agree to travel with them.
Back at the station, the clerk is turning out to be extremely strict and ruthless. Ying and the family shows up again, looking for Aang. Someone has stolen all their belongings, including their passports and tickets. Aang tries to get the clerk to give them replacements, but she refuses, complaining that she can't bend the rules. With no other choice for the family, Aang gives up their tickets and offers to guide them through the Serpent's Pass.
The group reaches the pass, which, seemingly against its name, is very straight, but very narrow. Much of it is barely wide enough for people to pass through single file. Ying seems to lose hope, and Aang says that perhaps it is best that they do abandon their hope, and instead focus on the present need to cross the pass.
That morning, Aang's group comes to a large setback: a portion of the pass is currently underwater. Katara steps forward, parting the waters to allow them to pass. As the path gets deeper, Aang helps her, forming a bubble of air for them to walk underwater safely. As it turns out, Serpent's Pass is named (and feared) not for its shape, but for the gigantic sea serpent that swims in the surrounding waters. The serpent attacks the bubble, forcing Toph to raise the sea-floor under their feet. This strands them in the middle of the water, with the serpent circling around. Aang attacks the serpent, telling Katara to get the others across while he distracts it. Katara creates an ice bridge for the others to cross.
The group finally crosses the Serpent's Pass, just in time for the mother to go into labor. Katara, who has had experience in birthing children, quickly takes charge. The baby, a girl, is born with no complications, thanks to Katara. Aang, seeing the family coming together, renews his hope, which is what the family decides to name the baby, Hope.
Ying reaches Ba Sing Se with her family and is now living in the Lower Ring. They enter the city on the same monorail car as Zuko and Iroh. Iroh comments on how beautiful her newborn baby is. She is later seen grieving with her family as she watches the Fire Nation occupy Ba Sing Se. The War ended with the city's liberation so it is safe to assume that Ying is finally enjoying peace with her family.