Lizzie Devine is Numbuh 1's chubby and overbearing girlfriend. She first appeared in the self-titled Operation: L.I.Z.Z.I.E., in which she uses a mind control helmet on her boyfriend in order to stop him from ditching her to spend time with the Kids Next Door. In subsequent episodes, Lizzie is largely a neutral character. Lizzie is similar in appearance to one of the Delightful Children From Down The Lane (which is Ogie - the girl with braided brown hair. This was only referenced in-series in Operation: C.A.K.E.D.-T.W.O., when she was bribed with the promise of a taste of the frosting on the Delightful Children's cake to dress up as one of them. She is also terrible at cooking, such as making a pie that knocked out Numbuh 2 and Numbuh 3, and soup that almost poisoned Numbuh 1.
Lizzie can be very possessive in concern of her boyfriend, and is very demanding and sometimes threatening of "Nigie" when she wishes to spend time with him, although he is usually occupied with more urgent matters. This behavior frequently causes problems for Numbuh 1 and the rest of Sector V during missions. Once she was so frustrated she even had the nerve to say "SHUT UP!" to the Delightfuls.
In Operation: D.A.T.E., she earned the temporary position as Numbuh 49 when Numbuh 1 told her coming to the dance was a mission. She appeared completely oblivious to the Delightful Children From Down The Lane's brainwashing of all the party guests and insisted on having her picture taken even after it became obvious that the camera was a brainwashing device. After that, when Nigel hurts her feelings and said that it was never a date and didn't care if she thought so, and was seen sad after Nigel slammed the door right in front of her face. Right before the Delightfuls were about to delightfulize Nigel, Lizzie stepped in with a glass of cherry punch from the party and shouted, "NIGEL UNO! I'D LIKE A WORD WITH YOU!". The Delightfuls asked what the meaning of this was, but Lizzie was so angry and just replied rudely and loudly, "SHUT UP!". After Lizzie gave a speech to Numbuh 1, she tried to wet him with her punch, but he jumped out of the way and the punch landed on the master fuse box. Numbuh 1 grabs Lizzie and hugs her, telling her that she's brilliant. She replies by saying, "Now, was that so hard?". Outside, the mansion blows up into fireworks as they and the Delighfuls run outside as the mansion is destroyed because of Lizzie's glass of punch.
Numbuh 1, while frequently annoyed by Lizzie's antics, deeply cares for her and usually makes up with her for whatever animosity has ensued at the end of the episode. However every other member of the KND tend to groan everytime she appears.
At the end of Operation: G.I.R.L.F.R.I.E.N.D., Lizzie breaks up with Nigel after he leaves her several times during their date to attend to the imminent danger present to them, courtesy of the KND Splinter Cell. Also, she was tired of Numbuh 1 always putting the Kids Next Door before her and neglecting spending time with her.
She was last mentiontioned in Operation: T.R.E.A.T.Y. by Numbuh 3, mentioning to Nigel at a rather bad time that Lizzie dumped him cold.
She also appears in the comic Operation: R.E.B.O.U.N.D., which takes place after she had broken up with Nigel. She enters the treehouse during an attack by The Delightful Childrens' Roboturkeys.