Arthur Watts is a former Atlesian scientist.
Arthur Watts is a member of Salem's group.
Not much is known of their relationship, but Watts appears to be loyal and respectful toward her.
There has been very little shown interaction between the two, but when Salem asks Watts to have a new tail made for Tyrian following his loss to Ruby Rose, he takes no issue with the request and sets out to perform it.
Professor Lionheart is Salem's mole in Mistral, and an important pawn in her targeting of Mistral. Arthur serves as the representative of Salem to Lionheart. Watts currently appears to treat Lionheart with civility but also occasional playful condescension.
As one of Ozpin's most trusted allies, Qrow is a primary target of Watts, who does not in any way underestimate the renowned Huntsman's strength and tenacity in combat. Due to this, Watts is extremely wary of Qrow's abilities and has no desire to combat him head-on.
He appears to look down on Cinder after she was defeated by Ruby. He greatly enjoys tormenting and angering her, especially since she is unable to respond following her injury. Given his comments, they apparently had a preexisting rivalry prior to her injury as well.
In "Dread in the Air", he has a disappointed tone after discovering that Cinder's voice has recovered. In "A Perfect Storm", Watts works with Cinder to pressure Raven Branwen into having Vernal use her Maiden powers to open the Relic of Knowledge's chamber. When she agrees to the bandit's demand for her brother's death, Watts warns Cinder of her intentions to get revenge on Ruby, however, she dismisses him and burns his hand in response.
Watts treats both Emerald and Mercury with disdain, derisively calling them Cinder's "posse".
Upon finding and meeting Raven Branwen in "A Perfect Storm", Watts is professional in front of her, but pressures her into cooperating with Salem. However, he disagrees to her terms for her brother's death because of the risks of exposure and putting the Kingdom on high alert.