YoonYul shippers are known as Twin Troopers, because Yoona and Yuri look very alike.
Yoona has a habit of making a cute angry face whenever Yuri is sad/angry, because Yuri is the member she relates to the most and she doesn't like seeing Yuri upset.
Both Yoona and Yuri have similar aftereffects when they get drunk. When YoonYul get drunk, both of them will say in a very high-pitch voice while waving their hands, "Aniyo aniyo! No no no! I'm not drunk! Jinjja! I'm not drunk, I'm not drunk!"