At first glance Oncelette is a female version of the Once-ler that hangs around with a boyish entity known as the Greed-ler, through out her endeavors she encounters many treacherous situations and dangers while also trying to do what's right for others around her as well as herself.
"In the future, Once-lette is Greed-ler. But the Lorax decides to give her a second chance so she’s sent back to the start of her journey but with no conscious memory of before. Her memories of the future reside with the imagined Greed-ler who has taken on his own physical form. But her second chance comes with a catch, 'And as the trees fall more, the stronger their desire or the stronger their power around her, to endanger her more.'
Greed-ler and Oncelette must spend their time rebuilding their ruined truffula valley, before Greed-ler and Oncelette’s curse takes its toll." (x)