Emerald and Yang first unofficially met in volume 2 of RWBY. By the first episode of volume 3 the two appear to be close friends. Yang even referring to Emerald as "Em" when greeting her. Later in volume 5 when they see each other again, Yang gets visibly furious. Emerald also later comes to help Mercury during his fight with Yang.
The two do not see each other again until volume 8. After yang, Ren and Jaune are reunited with Oscar, he explains to them that Emerald is on their side now. Yang then immediately gets angry and inquires why they should trust her. Emerald then retorts and that she knows how to get out of the whale. Yang and Emerald then proceed to escape with the others.
While walking back to the Schnee Manor they confiscate Emerald's weapons and have theirs pointed at her, all the while Emerald has her hands up. While arguing over what to do with Emerald, Yang yells at Oscar that she's part of the reason why she has a fake arm and that she's not going to forgive her. Oscar then tells yang she doesn't have to forgive her just to give her a second chance since she was on their side now. Emerald then tries to convince them to let her go so they never see her again. Later when the group fails at trying to think up a new plan, Emerald taunts them.
Yang then angrily asks her if she has a better plan she would love to hear it while walking up to her. Emerald then moves to draw her weapon before Oscar calms them down. Then later when Emerald pleads for them not to fail since she only now just joined them, Yang can be heard audibly going "aww!" in the background.
Mommy Issues is a rare pair within the RWBY fandom. Mainly because the two were enemies up until recently. Some think the two might have a similar relationship to Zuko and Katara from Avatar, since Katara had a hard time warming up to Zuko and Yang in turn is having difficulty warming up to Emerald. They might also be able to be able to bond over their rather blunt and brash attitudes. Also their shared pained experiences with their mother figures. It has a small fanbase on Tumblr.
- "Mommy Issues" comes from the fact that Yang has mommy issues with her own mother, and Emerald views Cinder as a mother figure and has her own issues with her.