Pre-Crisis
A metahuman with multiple personality disorder who has the power to absorb and dispel electricity in the form of lightning bolts. As such, she can cause the Marvels to revert to their non-powered forms if she attacks them with enough of a charge. She was created by E. Nelson Bridwell, Don Newton and Steve Mitchell.
Post-Crisis
When re-introduced in the Power of Shazam! series, Chain Lightning was depicted as a teenage girl, whose four personalities (her “actual” persona of Amy, the embittered Amber, her inner child, and her id) fuel her powers. Chain’s Amy personality has an unrequited crush on Captain Marvel, Jr., after he saved her (and “the others”) from committing suicide at the Statue of Liberty. During the ensuing conflict, Captain Marvel Jr. accidentally strikes her with his magic lightning, resulting in her alternate personalities briefly being visible. The personalities bond with the magic lightning and feel what its like to physically exist for the first time.
Following this, Amy is taken into care courtesy of STAR Labs. There she’s given care for her mental health issues and, after many months of rehabilitation is released. Immediately following her release, she finds a newspaper reporting on the Marvel Family getting Medal of Honors following the Fairfield crisis, with emphasis on the implied romance between CM3 and Mary Marvel. This pushes Amy over the edge again and she stalks CM3 to Fawcett City. There, she hides away in his car. After attempting to take control of the car, it goes careening off of a cliff. The Marvel Family attempts to transform, but Amy steals the magic lightning, resulting in them being unable to transform.
The magic lightning separates Amy’s four personalities into physical entities. Amber, an aspect repainting anger and wielding the Power of Zeus courtesy of the magic lightning, begins tearing through Fawcett City, resulting in many deaths. Windshear and her father Bulletman attempt to take her down, but Amy ends up fighting against her and in the end, they merge back together.
ID and the Inner Child seek out Freddy Freeman in the hospital, seeking to kill him for ‘betraying’ Amy. Caitlin Rousso, Amy’s doctor at STAR Labs, attempts to talk them down, but fails. The two kidnap Freddy. Mr. Scarlet intervenes before the two can kill Freddy, and while the two escape, Freddy tells them to go ask Amy is she really wants him dead.
A bit after this, Amy reappears whilst Captain Marvel is fighting Superman. It’s implied but not confirmed that her stealing the lightning temporarily corrupted it, causing Marvel to act erratically. Amy voluntarily gives herself up, allowing STAR Labs to take her back into custody.