While Mera primarily battles evil alongside Aquaman in the comics, she’s also been a member of the Red Lantern Corps and the Justice League United.
Along with underwater breathing and enhanced strength, Mera can also manipulate and shape water with her mind, an ability that all her people, the Xebellians, have.
No official details have been provided about Mera’s role in Aquaman, but like in the comics, we can presumably expect her to fight alongside Arthur Curry and eventually become Queen of Atlantis once he becomes king, if he isn’t already.