A Mangrave as well as an Imperial Wizard and Count who is a benefactor and sponsor for Emilia's bid for the throne. He is a highly influential individual who is also one of Lugnica's powerful mages.
Ambiguously Gay: Roswaal has a rather peculiar sense of fashion. Not to mention how he acts to Subaru. Somewhat justified due to living entire lives in the bodies of women.
The Archmage: Roswaal has access to all 6 of the main elements where as an average person has only access to one or two. He is the strongest non-witch magic user in the kingdom. This is because he was trained by the Witch of Greed before she died.
Aristocrats Are Evil: A Margrave of Lugunica as well as the strongest court magician. He is also the one who sends Elsa to steal the insignia, thus setting off the plot.
Body Snatcher: Roswaal steals the bodies of his descendants in order to complete his goal of reviving his master.
The Chessmaster: Roswaal is the direct cause of many of the early events of the story, like the theft of Emilia's emblem to the hiring of Elsa. He does this to recruit Subaru and his ability to help him resurrect his master, the Witch of Greed. However, he is played by Subaru of all people in arc 4 after both copies of the Witch of Greed's Gospel of the future are discovered and burned, forcing him to rely on Subaru to revive his master.
Foil: To Subaru. Both have women in their lives that are extremely loyal to and will do anything for, Emilia for Subaru and Echidna for Roswaal. In addition they both will not let death stop them, Subaru with Return By Death and Roswaal stealing the bodies of his descendants. The difference between them is that Subaru doesn't rely on morally wrong choices to achieve his goal.
Hidden Villain: Roswaal is the direct cause of the early incidents of the show that influence Subaru to join up with Emilia.
Impossibly Tacky Clothes: Roswaal could easily pass off as a court jester. [[spoiler: Upgrades to Fashion-Victim Villain once it is revealed that he is the one behind much of the first 2 arcs.
I Need You Stronger: Roswaal sets up the events of the first 2 arcs as well as parts of the forth in order to make Subaru more determined and "complete" him.
Large Ham: Roswaal loves to talk and dress larger than life. Becomes Evil Is Hammy upon being discovered by Subaru as one of the hidden villains of the story.
Love Makes You Evil: Love is the only reason for his mission of reviving his master.
Man Behind the Man: Roswaal hires Elsa in order to get Subaru to join Emilia's side as well as the events of Arc 4
Mark of the Supernatural: He has heterochromic eyes, the right one being blue and the left one being yellow. He's a Body Snatcher who has lived for many years by stealing the bodies of his descendants.
Token Evil Teammate: Forced into truly joining Emilia's cause in order to gain Subaru's help.
The Unfettered: Nothing will stop Roswaal from achieving his goal of resurrecting his master, even if it means killing or ruining the lives of multiple people. Not even death will stop him as he has stolen the bodies of his descendants to live for centuries.
Villainous Breakdown: After Subaru manages to Out Gambit him and solve the events of arc 4 without following the events written in Echidna's Gospel of prophecies as it fulfills a part of a contract with Ram, which states that she will serve him with her life as long as events followed Echidna's Gospel, but if this ends, then Ram could do whatever she wanted with him.
Walking Spoiler: Just look at the white text.
You Gotta Have Blue Hair: He has indigo hair.