As a founding member of the famous team of adventurers called the Fantastic Four, the Invisible Woman (originally known as the Invisible Girl) was a longtime friend and ally of the Avengers for many years before joining their ranks. 

     As her alias implies, she can render herself and other matter invisible, and can also generate invisible mental force. 

     When she and her husband, FF leader Mr. Fantastic, took a leave of absence from their team to spend more time with their son, Franklin, the child was abducted during the demonic invasion of Manhattan known as Inferno. 

  
     Captain America led them and other heroes in averting the crisis and rescuing Franklin, and then invited them to join his new Avengers team (the team's eastern roster had temporarily disbanded, so they were sorely in need of recruits). Convinced that they could no longer hide from their lives as adventurers, the couple accepted Captain America's offer but served only a very brief membership before returning to the Fantastic Four (partly because Mister Fantastic challenged Captain America's leadership too frequently). They did, however, agree to serve with the team as reservists of sorts for a time. 

     More recently, Invisible Woman, Mister Fantastic and their fellow Fantastic Four founder Thing all declined further formal participation in the Avengers, deciding to concentrate more fully on their work with the Fantastic Four; however, while they may no longer be Avengers in their own right, the Fantastic Four stand ready to assist the Avengers whenever they are truly needed. 

Invisible Girl: 
     First alias employed by Susan Storm (later Susan Richards) in the early years of her career as a costumed adventurer. She later changed her alias to Invisible Woman, wanting a more mature title. As the Invisible Woman, she served briefly with the Avengers.