Dagger

Origin

In Secret Invasion #1, Luke Cage calls Cloak, who drops the New Avengers at the top of Stark Tower to steal one of Tony Stark's quinjets. When Cage offers to take him with them to find the downed Skrull ship, Cloak refuses, and vanishes.

Dark Reign

Dagger was approached by Norman Osborn who offered her a chance to have her criminal record sealed, only if she went along with joining his Dark X-Men. 
 
She and Cloak accepted, and became incorporated into Osborn's personal team of X-Men. Fighting alongside stalwarts like Emma Frost and Namor, Dagger performed exceedingly well, coordinating well with the rest of the team as they helped to keep the peace in San Francisco. Siding with Osborn didn't sit well with the young heroine or her partner, and she once more felt the urge to flee from the situation. Cloak stopped her, saying that if they ran away again, they would always be running. She and Cloak stuck around, despite witnessing some of the more unsavory practices of Osborn's team. Thus, when Emma and Namor betrayed Osborn, revealing their siding with the HAMMER director as part of Scott Summers' ploy, both Dagger and Cloak were invited to join the true X-Men in their exodus from San Francisco.
 
Tandy and Ty accepted this  more readily than they accepted Osborn's entreaties, and thus the two runaway mutants were finally part of a team, and among their own kind.

Powers

As a mutant, Dagger has the ability to create psionic light daggers which travel wherever she wills them, and which drain living beings of vitality ("life-force") when struck. Her light daggers also have the capacity to cure certain persons of drug addictions, and can alleviate Cloak's hunger for light. Cloak and Dagger both have moderate experience at street-fighting, and Dagger's combat techniques utilize her light powers and ballet dance training.  Her powers can be used for a variety of effects:

• Living Light Generation: Dagger generates a form of living light which is actually life-force. 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 life-force 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 Dagger: 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 life-force disrupted. Dagger can then drain a portion of their life-force; 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 etc.
  
• Living Light Manipulation: Nonliving objects are unaffected by her daggers; 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 life-force energy in a diffused manner, allowing her to illuminate dark areas as if by natural light. Tandy can also project 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.

Alternate Realities