Keep in mind this post will only cover the first season of the anime.
The duo first interacted when Christa offered an exhausted Potato Girl (Sasha) bread when Ymir questioned Christa’s unnecessary kindness but quickly noticed Sasha being easy to manipulate.
During a surprise attack from the Titans later on we got this scene after Christa stopped Ymir from belittling Armin and arguing with baldie.
Then they do some random stuff once in a while. Ymir at one point during the second major arc was standing on a giant tree’s branch keeping an eye out for incoming Titans when the first sentence she uttered was “Has anyone seen Christa?” or something like that.
That’s the gist of their interactions in anime really. If readers are not one of the many who could not wait and immediately picked up the manga for more YumiKuri goodness then you will have to wait till 2016 for the second season, which I will do as I am in no rush to watch more AoT.
PS: I forgot to mention that even though the second image was initially a joke YumiKuri was eventually confirmed as canon by the creator so something more to look forward to when they’re on screen or in a manga chapter.