Kissing Cousins: It is revealed that his father was the son of a prostitute and a noble of House Capulet, meaning that Romeo and Juliet are actually related to each other, although how closely is ambiguous.
Lonely Rich Kid: Romeo is hinted to be one, since Benvolio seems to be his only friend and all other nobles look at him with different degrees of contempt.
Romeo's father, and the man who rules Neo Verona with an iron fist. He was the one who murdered Juliet's parents fourteen years ago, starting off the story. It's revealed that he's an illegitimate child of a Capulet man and a prostitute, and swore revenge against his father's family after his mother died.
Axe Crazy: Even at the best of times, he can barely hide the fact that he's rather psychopathic, stripping Benvolio's father of all of his titles because he dissented against him. After the Capulet rebellion, he goes off the deep end completely, killing two of the members of the council because the rest wouldn't support his proposal to BURN DOWN THE COMMON PEOPLE'S QUARTERS
Freudian Excuse: His hatred against the Capulets was because his father was one and abandoned him and his prostitute mother to a poor life. After his mother died from sickness, Laertes swore revenge against his father and his family for not taking care of them by killing all of them. Yikes!
Hypocrite: His entire motivation was a Capulet man using his mother and tossing her aside, condemning him and her to a life of poverty. Yet this was exactly what he did to his original lover and his illegitimate son, Tybalt.
Kill It with Fire: his proposed solution to the Capulet rebellion. Notably, only Mercutio actually supports him in this until he pulls out his sword and kills two members of the council right in the middle of the chamber
Demoted to Extra: Compared to the rest of the characters that are adapted straight from Romeo and Juliet itself, Mercutio's role in the series can come off as this. A much straighter example would be his appearance in the manga, wherein he has no plot relevance whatsoever due to the changes made to compress the story.
What Happened to the Mouse?: The manga, only spanning two volumes, actually cuts out and rearranges (with very little effect) whatever plot relevance he had left. He's basically there to seethe about Romeo, and unlike many of the other surviving characters, it's never revealed what happened to him afterwards.
Barrier Maiden: Escalus, the tree which holds Neo Verona, is dying and as the daughter of the Capulet family, Juliet is the seed of the Tree and she must sacrifice herself and merge with the tree in order to save Neo Verona.
Compressed Hair: Juliet's hair is waist-length, but she has no problems concealing it under the short-haired "Odin" wig. Justified, as it is rather easy concealing it. Just put it in a French twist, pin the wig in place, and everything's fine.
Cycle of Revenge: he wants to kill Laertes Montague for almost the exact same reasons that Laertes wanted to kill the Capulet family, (specifically, for refusing to recognize them as legitimate sons and causing their mothers' deaths)
Kissing Cousins: Subverted. He looked like a possible romantic rival to Romeo (who even mistook him once as Juliet's lover), but he doesn't feel like that for her. In fact, he becomes a Shipper on Deck later...
A miner who looks out for Romeo during his stay at the mines. While he admires Romeo's idealistic dream of creating a world where people can live together without sacrifices, Giovanni also believes that Romeo has chosen a long and difficult path to pursue.
A mysterious yet sage man who frequently smokes from a long pipe, he is suggested to bear some connection to Escalus. He acts as Romeo's spiritual adviser, mentioning that the noble will have to someday shoulder Juliet's fate.