Jean Loring was the ex-wife of Ray Palmer a.k.a. The Atom. She was the catalyst behind the events of Identity Crisis, though this isn't revealed until the end. Later on she would become the next Eclipso.
Jean and Ray had a difficult time trying to make their marriage work with Ray's double-life as the Atom constantly calling him away from her. This led to Jean developing a slight resentment towards the other spouses such as Lois Lane and Sue Dibny, who somehow seemed to effortlessly manage. Ultimately they decided to divorce.
Sometime later, Jean wanted to get back with Ray. To do this she used one of his shrinking-devices to crawl inside Sue Dibny's head and give her a minor stroke, the idea being that news of this would be enough to make Ray want to reconcile. The plan worked a little too well as Jean accidentally killed Sue by causing an irreparable disruption in her brain. The events led to an investigation as to who could have done such a thing, causing a cold war between superheroes and villains. In the calamity, Ray and Jean get back together after Ray discovered Jean was nearly hanged to death (this was actually Jean's own doing to avoid suspicion).
Jean then has Captain Boomerang attempt an assassination on Robin's father, though she provides him with a gun and a warning to avoid actually getting him killed. After Jean accidentally lets Ray know that she knows about Robin's father being murdered (information that Batman kept confidential), Ray deduces that she had a part in his assassination and as a result that she was Sue Dibny's killer.
Jean is then incarcerated in Arkham Asylum.
As EclipsoWhile committed in Arkham, Jean discovers a black stone that transforms her into the new Eclipso. As Eclipso, Jean was able to control the Spectre and do battle with several magical beings. This backfired on Jean and led to her being permanently trapped in the Sun's orbit.
Sue's husband the Elongated Man uses the Helmet of Fate to give Jean a chance to see the error of her ways and punish her accordingly, but when he sees how remorseful she is, he realizes he isn't ruthless enough to punish her, so he leaves her tuck in orbit as Eclipso's host.
Eclipso manages to escape the Sun's orbit and attempts to possess Mary Marvel, but is separated from Jean, who drowns in the ocean.
Jean was originally a gifted lawyer and one of the last people who could be expected to turn to villainy, but when her marriage to Ray failed her sanity began to take a turn for the worse. Jean began taking dangerously high risks just to get Ray to notice her again, not even considering that maybe all she had to do was call Ray and reconcile. Her recklessness costs Sue Dibny her life, shaking Jean up even further as she then sets up several fake assassinations on superhero's loved ones in an attempt to make it look like a serial killer is on the loose, with herself as a target. When Sue finally gets her come-uppance, several superheroes reveal that they don't hate her for her actions, but rather pity her as she was simply a lonely ex-wife rather than a true monster.