The only way to reverse Eileen's transformation is to persuade her to do something generous instead of something greedy, a trick discovered by Violet in the episode "Birthday Girl", and used several times since then.
She thinks that every day is her birthday, and such uses the excuse as a reason to get special treatment from anyone and everyone, usually to great effect. Because she thinks every day is her birthday, she thinks every party is her birthday party, every cake is her birthday cake, etc.
Eileen also greatly annoys Becky because of her lisp, using the "w” sound in words instead of “l” or “r”. e.g. “I wearned my wesson.” "Today's my biwthday!" "What do bestest-westest fwiends do?"
In addition to using it to get what she wants, she also uses her cute charm to get herself out of trouble, as shown in "The Wrong Side Of The Law".
In "Who Wants Candy?", Eileen wants to be best friends with Violet and her field trip buddy.
History of The Birthday Girl
When being chased by B&M 1732, cat burglar Tammy, is cornered, and hides in a toy store. Before breaking away, she finds boxes of talking "The Girl Who Love" dolls, and uses a voodoo ritual to bring one of them to life.