Little of Miri's background story is known prior to her joining the Star Sapphire Corps.
While on an interstellar honeymoon with her new husband, Kered, their space ship was attacked by by Mongul (who, at that point, was in possession of several Sinestro Corps power rings).
Mongul had just escaped the Black Mercy planet and came after them only becasue he was hungry and wanted the food on their ship. Kered was killed by an errant swat of Mongul's finger when he tried to make peace with him.
On the verge of death and with a great sorrow in her heart, Miri wanted to kill herself to be with her lover.
A Star Sapphire ring came to her then, and told her she had great love in her heart and that the universe needed her to share that love with others - to protect love, and the hearts of those who love. She accepted the ring's offer and joined the Star Sapphire Corps.
From there, she traveled to Zamaron and trained as a Star Sapphire with the Zamarons and, presumably, other members of the corps.
She completed her training and was sent off into the stars by the Zamaron queen, Aga'po.
As a relatively recently introduced character Miri has not participated in many story arcs, but those to which she has been part of have been significant in importance, as well her importance to them. Her first major appearance was in the story arc Sins of the Star Sapphire. (though it is worth noting that that title turned out to be something along the lines of false advertising, as, during the storyline, neither Miri nor the Star Sapphire Corps did anything whatsoever that could actually be at all accurately termed as sinful - quite the opposite in fact). She was involved in stopping Kryb from stealing the children of Green Lantern Corps members (specifically Amnee Pree and Matoo). She is later seen in during the Blackest Night. Kryb constantly hears the voices of the children she stole, alone and dying. Miri detects that although they aren't hers, Kryb does in fact feel deep love for the children. Unaware of the coming battle on Zamaron, Miri escorts Kryb to her children. After a long flight, Miri and Kryb find the cave empty of all children. Miri scans for heartbeats, detecting only hers and Kryb's.
Miri suddenly receives a message that a great love is dying on Oa, and she takes Kryb and teleports there. On Oa, Soranik Natu stands in a crater with a dead Kyle Rayner. Kyle detonated a fellow Lantern's power battery, destroying hundreds of Black Lantern's. Guy Gardner fights off a Black Lantern Ring from making Kyle one of them. Miri and Kryb arrive on Oa, and Miri rushes to Soranik and Kyle. Miri refuses to let their love die, and uses the love inside Soranik's heart to reach out to Kyle's, bringing him back. They then realize the situation has worsened around them. Kryb's children have died and were resurrected as Black Lanterns, and Guy Gardner has received Vice's Red Lantern ring after being enraged from Kyle's death. The Black Lanterns begin destroying the Central Battery, as Guy burns through the Black Lanterns. Kryb tries to stop Guy from burning her children, and Guy rips of two of Kryb's arms and spits Red Lantern plasma into her throat.
Powers and Abilities
As do all members of the Star Sapphire Corps, Miri wields a Star Sapphire Ring, which operates very similar to a Green Power Ring. She has the ability to create hard light constructs, fly, and survive in outer space among other abilities.
Miri is also one of the Star Sapphire Corps members who is seen to be the most capable at her duties.
Though relatively inexperienced she has survived some of the most dangerous missions that the Corps has undergone.
Star Sapphire Power Ring
Flight and Space Travel: She can surround herself in an energy field and fly within a planet's atmosphere or through outer space. In space, she can generate an interstellar space warp to travel between solar systems at great speed.
Energy Blasts, Shields, and Constructs: She can fire energy beams with destructive power, generate energy shields for defense, and create energy constructs that can take the form of anything she can imagine. The power of these is proportional to her ability to tap into and believe in the love within her heart.
Sapphire Crystals: She can surround others in a prison of sapphire crystal to show them what they love most, as well as bring out the love that may be buried in their hearts.
Heart Tethers: A heart tether can bring her to someone she loves or guide her to where love is threatened out among the stars.
Teleportation: She is able to teleport herself and others on an interstellar scale, provided she can harness enough love to power it.
Healing: She can use the power of love to heal even normally fatal wounds. Conceivably, she could also even bring someone back from the dead.
Heart Prophecies: She can use her power to look into someone's heart and show them a vision of their future.