Yep she's usually just considered a background player and supporting character with Aquaman. Where Mera should be with Aquaman like the Invisible Woman is with the Fantastic Four, Mera is really pretty much DC's whipping girl. You know those DC writers are kinda sick. But...she is a redhead....
Originally Mera was the Queen of Dimension Aqua (aka Xebel). When her kingdom was seized she fled here and met Aquaman and Aqualad, here in this dimension, they helped her, Aquaman and Mera got married. Later they had a kid, Arthur Curry, Jr., aka Aquababy. Yeah I know....I know....
Of course DC has hacked up her origin to make her more "grim, gritty and edgy" now. DC really needs to chill.

Black Manta kills Aquababy. Yep...they did that. Mera gets put in an insane asylum in Atlantis. She escapes the asylum and Aquaman accidentally kills her. Mera gets better and goes back to Xebel.
Then they change their minds again... Mera wasn’t a good guy escaping Xebel, she was a trained assassin sent to kill Aquaman. But she feel in love with Aquaman and they had a kid, kid died, Mera went crazy, etc. Her people were actually the ones that created Kaldur'ahm (Young Justice's Aqualad) by breeding Black Manta and a woman.

But thanks to The New 52 relaunch...none of that ever happened! So it's all cool now. Actually Aquaman and Mera were one of the things DC DID do right in the relaunch. You actually get the sense with her, that she IS the great woman behind a great man.
Let's just move on to her powers okay?

Mera has Aquakinesis or Hydrokinesis. That means she can increase the density of water and then create any structure with it. Swords, knives, whips, battering rams, kinda like Green Lantern and his Power Ring constructs. She has limited telepathy, super strength and the ability to travel to Dimension Aqua (aka Xebel).

Outside the comics, she fairs MUCH better on TV. She appeared on The Superman/Aquaman Hour of Adventure television series in 1967, Justice League in 2001 and in Batman: The Brave and the Bold in 2008. Her best animated appearance I think was in Young Justice where she was Aquaman's wife, the Queen of Atlantis AND the head of Atlantis Conservatory of Sorcery (think underwater Hogwarts). And we had the very hot Elena Satine show up as Mera in the final season of Smallville.
They were all MUCH happier than the comic book version.

Mera - DC Comics