First Appearance: Sun Girl #1 (August 1948).
Golden Age Appearances: Sun Girl #1-3, Captain America Comics #69, Human Torch #32-35, Marvel Mystery Comics #88-89.
Modern Appearances: Saga of the Original Human Torch #3-4.
Years Active: 1946-?
During the Golden Age Sun Girl's private identity and origin were never revealed; she fought the bad guys and replaced Toro as the Human Torch (I)'s sidekick.
Well, okay, it's not quite as brief or simple as all that. While her origin was never revealed, some things about her were known. She had friends in the scientific community she could call upon at any time. Oh, and she's also, according to Sun Girl #3, "the nation's most popular personality," so famous and popular that when a giant ape-creature from Brazil is loose in the city, an Admiral says, "I would suggest the atom bomb rather than sacrifice the life of Sun Girl!" Her efforts to capture the animal are broadcast internationally and listened to with great interest across the world, from China to Russia to South America to the North Pole. When Sun Girl succeeds in capturing the beast, schools across the United States are closed for a day in her honor. Some textual evidence indicates that she'd been active for more than the short amount of time her stories credit her with; she is seen to be familiar with a mad scientist who is being released from prison after a long stretch in The Big House, and one story in Sun Girl #1 is told narrated by a grown man and judge, who was rehabilitated, as a teenager, by Sun Girl, thus implying that either the story is set in the future or that Sun Girl was active for twenty years or more.
In the modern era her background was changed and most of her adventures retconned away. In current continuity she was Mary Mitchell, the personal secretary of Jim Hammond, the Human Torch (I), and that she knew all about Hammond's other life; she'd fallen in love with him and became his secretary to get close to him, and when Toro left the Torch to tend to his ailing foster mother she leapt to replace him. When Toro returned he replaced her as the Torch's sidekick, and Mary was left behind, never seeing the Torch again.
Sun Girl was a former WAC, an acrobat and judo & jiu jitsu expert and used a "sunbeam ray" against criminals; there are some slight indications that she might have had superspeed (although it might just have been her sprinting really quickly). She also carried an "emergency pouch" while on the job; the pouch had, among other things, a "portable electronic tracer."
Note: As Ronald Byrd says, "THIS is the woman that Roy Thomas blithely dismisses as the Human Torch's secretary." Sun Girl must be counted as one of the most victimized characters of Roy's Retcons, I think.