In the comics, of course, Brigid and Mayhem are one and the same. During a bust at a warehouse in 1983's Cloak & Dagger #5, the corrupt cops in Brigid's department led her right into a trap. When she discovered a piece of evidence in plain view, she approached with two other cops, only for a plastic cage to emerge from the floor and trap them inside. What's worse, the plastic cage turned out to be a gas chamber. Gas immediately seeped in through the floor and, despite a few valiant efforts to escape, all three officers suffocated to death.
Nevertheless, this wasn't the end of O'Reilly. Cloak and Dagger discovered her too late to save her, but Dagger tried to resuscitate her anyway. Dagger's efforts brought something back, but it wasn't wholly O'Reilly. O'Reilly transformed into a creature with pallid skin and pupil-less green eyes; what's more, she now had strange new abilities, including the power to emit venomous gas from her pores, a scratch that paralyzes and flight. Following her transformation, Mayhem abandoned all the principles O'Rielly clung to as a cop, becoming "the ultimate vigilante."
Per Cloak & Dagger's Season 2 premiere, Brigid and Mayhem are two separate characters, though they have the same physical appearance. Like the comics, Mayhem's powers seem to rest in her fingernails, as she can be seen murdering Mikayla's abductor with her talons. The show's version of Mayhem also disregards the rule of law, resorting to violent methods to get what she wants. As such, Mayhem poses a major threat to not only Brigid's career, but also Tandy and Tyrone's mission to stop a human trafficking ring.
By splitting Brigid and Mayhem into two different characters, Cloak & Dagger sets up an opportunity for star Emma Lahana to stay on the series, should it be picked up for another season. After all, this allows the protagonists to defeat Mayhem while Brigid remains on the show as a main character. Unfortunately for the detective, though, Mayhem will be able to wreak a lot of havoc on her life because they share the same face.
Airing Thursdays at 8 pm ET/PT on Freeform, Cloak & Dagger stars Olivia Holt as Tandy Bowen/Dagger and Aubrey Joseph as Tyrone Johnson/Cloak, as well as Emma Lahana as Brigid O'Reilly/Mayhem, Andrea Roth as Tandy’s mother Melissa Bowen, Gloria Reuben as Tyrone’s mother Adina Johnson, Miles Mussenden as Tyrone’s father Michael Johnson and J.D. Evermore as Detective Connors.