Andal is the only female Alvar (poet-saint) out of the twelve that are associated with the Vaishnava tradition, in South India and also the adopted daughter of Periyalvar, another of the Alvars.
Periyalvar (or Vishnuchithan) one of the twelve Alvars of South India once found a baby girl in his garden. Accepting her as a gift from God, he raised the child as his own daughter. Due to Periyalvar's influence over the years, Andal also grew up to be an ardent devotee of Ranganatha (or Perumal).
Periyalvar had a daily routine of stringing together a fresh garland for the God. What he was not aware of however , was the routine of his daughter! Andal would first wear that garland herself, before adorning the god with it. One day, Periyalvar found out this secret, and admonished her for doing so. He made a fresh new garland for the God, but alas! it would not stay on the idol! That night, Ranganatha appeared in his dream & told him that he preferred the garland only after Andal had worn it!
In fact this practice is followed even today - the garland of Andal from Srivilliputhur Andal Temple (the original place where Periyalvar found her as a baby) is sent to Tirumala Venkateswara Temple on Garudotsavam and Azhagar Koyil.