Warner Bros. and DC Comics laid the groundwork of their DC Extended Universe with the release of Zack Snyder's Man of Steel in 2013, but the DCEU will officially launch with the follow up film, Batman V Superman: Dawn of Justice, next spring. Last year, Warner Bros. revealed their plans for the universe, with films scheduled through 2020 that include a number of standalone movies, headed by heroes like Wonder Woman and Aquaman, as well as team-ups including Suicide Squad and Justice League Part One and Two.
Although Justice League Part One won't be released until 2017, many of the characters expected to appear will be introduced in earlier movies. Wonder Woman (Gal Gadot) will be featured in Batman V Superman, with Aquaman (Jason Momoa) and The Flash (Ezra Miller) rumored to have cameo appearances. Additional rumors suggest an Aquaman easter egg will be included in Suicide Squad as well. Now, a new rumor claims a DC Comics character will make their debut in Justice League Part One before likely going on to appear in another DCCU film.
Umberto Gonzalez of Heroic Hollywood is reporting Mera, the Queen of Atlantis and Aquaman's wife from the comics, will be introduced in Justice League Part One - presumably before going on to appear in Aquaman in 2018. According to Gonzalez, his "talent agency sources [are] seeking background" on the character, with casting expected to begin soon. Gonzalez isn't sure if Mera will join the ranks of the Justice League or appear in the film in some other capacity, but she is "considered the Khaleesi of the sea and Aquaman's true love."
DC Comics and Warner Bros. are currently gearing up to begin production on Wonder Woman, which is scheduled for a summer 2017 release date, and earlier plans stated Justice League Part One would begin shooting in spring 2016 for its fall 2017 debut. The film will reunite Superman (Henry Cavill), Batman (Ben Affleck) and Wonder Woman, as well as presumably include Aquaman and The Flash.
Given that the DCEU has already confirmed it will introduce heroes in larger team-up films before they go on to star in their own features, it would make sense to do something similar with Mera. She could have a small part or cameo in Justice League Part One, then play a much larger role in the Aquaman film as "the Khaleesi of the sea" (a particularly funny description given Momoa's past work as Khal Drogo on Game of Thrones). Additionally, Mera's ties to Aquaman in the comics position her character as the obvious love interest/co-star for the King of Atlantis' DCEU solo feature.
However, this rumor should be taken with a grain of salt since it hasn’t been confirmed that Mera will appear in Aquaman, let alone be introduced in Justice League Part One. There are plenty of rumors circulating the DCEU, but DC and Warner Bros. haven't confirmed much, with even the full official roster of the Justice League still at least partially uncertain. That being said, even if Mera doesn't make an appearance in the first Justice League film, she could still be involved in the larger universe as a character in Aquaman. But, we'll need to wait for more confirmation before we can expect her to appear in the DCEU at all.
Batman V Superman: Dawn of Justice opens on March 25, 2016, followed by Suicide Squad on August 5, 2016; Wonder Woman on June 23, 2017; Justice League Part One on November 17, 2017; The Flash on March 23, 2018; Aquaman on July 27, 2018; Shazam on April 5, 2019; Justice League Part Two on June 14, 2019; Cyborg on April 3, 2020; and Green Lantern Corps. on June 19, 2020. The Superman and Batman solo films have release dates TBD.