The two were first introduced together alongside Emerald in the last episode of Volume 1. They are then seen onscreen together in volume 2, chapter 1. Although Mercury seems loyal to Cinder and always follows her orders, he doesn't seem to be as devoted to her as Emerald is. He also seems to to view her in high regard and has a great deal of confidence in her.
In volume 2 chapter 7, after Cinder successfully infiltrates Beacon and gets what she needs she then heads off to the dance and asks Mercury if she can dance with him. When mercury then asks what do they do now, she replies that they can just enjoy the rest of the evening since it is a party after all. It is then revealed in volume 3 chapter 7 that when Cinder first met Mercury, he had just killed his own father, an assassin who Cinder had originally wanted to recruit. She then decides to invite Mercury instead, despite Emerald's initial protests to which he gladly accepts.
After the Fall of Beacon, in volume 4 chapter 1, Mercury can be seen standing in the background showing some concern towards Cinder, since she has now lost an eye, arm and the ability to speak. He then later accompanies her to find raven branwen and then there eventual battle at Haven academy. After Mercury learns about her subsequent defeat at the Haven Vault, Mercury loses confidence in the mission and decides to retreat from the battle with Emerald and Hazel. Later in volume 6 chapter 4, he and Emerald are shocked when they learn that Cinder is still alive. In volume 6 chapter 9, he tells Emerald that Cinder was a pain, but that at least she always kept them informed. He then also told Emerald that the reason why he joined her and Cinder was out of pure convenience. He then also told her that Cinder does not care about her him or anyone else for that matter.
In volume 8 chapter 1, When Cinder returns to Salem's Faction, Mercury looks at her unimpressed and stays silent. In chapter 6, Mercury openly defies Cinder and tells emerald in front of cinder that she doesn't care about her. Cinder then angrily tells him to get out and tries to boss him around more. Mercury then smugly tells Cinder that he doesn't take orders from her anymore but Salem. This saddens Cinder quite a bit and she looks down in defeat. This also greatly angers and annoys Cinder since Mercury is now her colleague instead of her subordinate.
Thermometer is a rarepair in the rwby fandom. Mainly because the two dislike each other a lot and make that quite apparent to each other. Nevertheless it has a small fanbase on tumblr. Most people who do ship them though like the whole sexy evil aesthetic the two have. People who don't ship this usually ship more popular couples like Emercury, Emberald or even Combat Boots.
- "Thermometer" comes from the fact that thermometer's used to be filled with Mercury and they rise when it gets hot outside.