Tandy Bowen was a sixteen-year-old girl who grew up in Shaker Heights, Ohio, a suburb of Cleveland where many wealthy people live. Bowen's mother, Melissa, was a celebrated and self-centered model who had become rich not only from her own career but also from receiving her previous husband's estate. Tandy's father had left for India seeking "spiritual rebirth." Subsequently, Melissa married Phillip Carlisle. Although kind, Tandy was still hurt by her father's leaving and did not accept Phillip. Her mother also neglected her.
Feeling unloved, Tandy turned for affection to an older teenager named Rob Daltry. But when he left for college shortly thereafter, the despondent Tandy left home on a bus for New York City. While there, a man tried to rob her but was stopped by the homeless Tyrone Johnson. She bought him food and the two became friends.
Dr. Simon Marshall and his men rounded up runaways, offering them food and shelter. Naive, Tandy agreed, and Tyrone went with her, suspecting things were not right. There, they were knocked unconscious and injected with D-Lite, a designer drug the Maggia wished to use as a replacement for heroin. Because of latent mutant genetic factors and some magical interference by D'Spayre, Tyrone and Tandy survived while the others died due to the drugs' effects. They escaped, along with another prisoner, the Chinese human-carrier who would become Mister Negative, jumping into the river, and both began manifesting superhuman powers. They discovered Tandy could keep Tyrone in check. Marshall's men attacked, but were consumed by Cloak's hunger then hit with Dagger's light daggers.
Cloak & Dagger
The two became Cloak and Dagger, pledging to protect other young people from the dangers they had encountered. Tandy is fiercely loyal to Cloak. Even when offered to be reunited with her stepfather, she remained with Cloak.
Cloak was manipulated by Nightmare into wanting to consume all lawbreakers. Dagger sought the assistance of Daredevil, Black Widow, Moon Knight, Shang Chi, and Doctor Strange. Eventually, Dagger was forced to use her full power on Cloak, which seemed to transfer his darkness to her, reverting him to normal. Since then, Cloak has regained his powers from Dagger and the two continue to adventure.
Orderly Reginald Mantz managed to replicate Cloak's powers with Mutant Growth Hormone. He then beat Dagger and left her hospitalized. The Runaways and the New Avengers found evidence of "Cloak" acting this way and confronted the real Cloak and Dagger; however, they eventually discovered the truth. She and Cloak were able to regain their memories of the Runaways.
When Emma Frost and Namor eventually revealed their true allegiance, they ask Cloak and Dagger to stay back and, following the traitors on the newly build mutant nation of Utopia, were presented by Cyclops as being part of his plan from the beginning so they would be accepted in the community.
Later, she was one of those who convince Cyclops that his attitude toward Deadpool (rejecting him from joining Utopia's X-Men) was kind of hypocritical.
Cloak and Dagger's powers were switched thanks to Mr. Negative during the events of Spider-Island. Cloak now provides the light, while Dagger must feed from Cloak's light energy to temporarily satisfy her cravings. It was also foretold that Dagger would be the one to eventually kill Mr. Negative. To what extent their other powers have been exchanged has yet to be seen. 
Living Light Generation: Dagger generates a form of 'living light' which is actually lifeforce. She produces much more than normal humans do, but is usually conservative in the use of her powers. Overuse of her powers could be fatal. Dagger generates and stores this lifeforce energy automatically. If she goes for more than a week or so without using her powers she will become fevered and delirious, and eventually discharge her energy involuntarily.
Light Daggers: Tandy can form 6-inch 'light daggers' which she can throw from her fingertips. Her daggers may automatically form under certain conditions, such as another person in danger. The maximum effective range is unknown, but the daggers seem to weaken with further distance. Tandy has a limited ability to control their flight and reinforce them by compelling them to seek targets. They can track a being, but it is most effective when searching for Cloak. Those hit by these knives have their own lifeforce disrupted. Dagger can then drain a portion of their lifeforce; enough to kill if she wishes. Normally she chooses to leave them in a state of shock where they will see a vision of how their life might be different. The experience is powerful enough to make most individuals rethink their choices, although a hardened criminal or evil being is unlikely to change their ways.
Living Light Detoxification: Tandy's energy can also 'purge' a person, removing toxins, drugs, and other malignant substances from their system. It also has the capacity to cure certain persons of drug addictions.
Living Light Manipulation: Sometimes Tandy can redirect the daggers after they ricochet, other times the daggers are dispersed if they hit a non-living thing. She can project her lifeforce energy in a diffused manner, allowing her to illuminate dark areas as if by natural light. Tandy can also projects her 'light' into Cloak's dimension to feed his 'hunger.' She is unharmed when traveling through his dimension and can pass this protection to others.
The Civil War Battle Damage Report stated that the drug giving them powers in fact triggered latent mutant powers, but that they were science- and magic-based mutates.
When Norman Osborn came to recruit them for his X-Men, they stated they weren't mutants (to which Osborn replied they may as well be).
Dagger was 'confirmed' by Doctor Nemesis as NOT being mutant, with no X-Gene, no altered DNA helix or hereditary markers, no readings on Cerebro. But she mentioned that there was other more extensive tests that could be done on their bone marrow to indicate if they possess an X-Gene, which was suggested by Beast.
The biggest clichés in Cloak and Dagger stories: (1) They lose their powers, and then get them back later; (2) their powers are altered; (3) Cloak does something passive-aggressive to try to keep Dagger close to him, and it screws things up; (4) Cloak's powers go out of control; or (5) their powers are given to other people. Writers have trouble using the two characters without rehashing one or several of these clichés.
Daggers first love, Rob Daltry, is a reference to Robert Daltry of "The Who" fame. There are several "Who" references in their first stories.