Her obsessive crush on Numbuh 1 is incredibly creepy.
She's incredibly selfish, narcissistic, obnoxious, and painfully unaware of the KND's importance, so much to the point that she is a toxic girlfriend to Nigel.
More often than not, she's just there to distract Nigel from an important mission. All she cares is to date Nigel and nothing else, and the episode “Operation: D.A.T.E.” proves this all in black-and-white, especially when Nigel calls her out for being "date-crazy".
She's almost always shouting, often at the loudest volume, which gets irritating after a while.
Despite being marketed as "Nigel's Girlfriend", she's almost always rude towards him.
She's a dreadful cook. As she once managed to bake a pie that knocked out Hoagie, Kuki, and Numbuh 1-Love, and a chicken banana fritter soup with cottage cheese, mud, and curly fries that almost poisoned Nigel, as well as making some strange dish out of oat bran, cream, tomatoes, steak, orange juice and tons of salt that lit her mouth on fire and is actually proud of her own cooking.
Nigel and the rest of Sector V had no hard feelings over the little matter of her having originally tried to brainwash him.
Her temper is so bad, that even Numbuh 4, who is practically the grouchiest and most violent member of Sector V, doesn't admit his dislike for her due to his fear of said wrath.
In “Operation: L.E.A.D.E.R.”, she goes behind her boyfriend's back and leads Sector V on a mission, which in reality is just them going out for ice cream and a makeover, only for them to fail miserably without Numbuh 1. On top of that, in that episode, she proved to be a terrible leader for Sector V, to the point that Numbuh 2, 3, 4, and 5 turn on and bicker at her while on the journey for their mission, all because she told Numbuh 4 to shut up.
In “Operation: H.O.L.I.D.A.Y.” she gets jealous when she hears that Nigel is hanging out with Numbuh 10 (who is considered the prettiest female operative at KND) at a Jamaican beach in Sector J, which Lizzie tags along with Numbuh 2 and 3 in their hamster airline vacation and her horribly cooked pie that she baked made Numbuh 2 and 3 unconscious and made them crash land on the beach, then when she finally arrives to scold Numbuh 1, Numbuh 10 reveals that she and Nigel are cousins, and at this point, you'd think she'd realize she overreacted and should apologize to him, but no, instead she accuses Numbuh 1 of trying to date his cousin (which he obviously isn't) and she ends up bonding with Numbuh 10 over Nigel's flaws and Lizzie gets off scot-free for the damage. In short, she falsely accuses Numbuh 1 of committing incest when he actually isn't!
In “Operation: C.A.K.E.D.-T.W.O.” she was actually working for the Delightful Children From Down the Lane against Nigel. What doesn't make sense is that she's supposed to be his girlfriend.
In “Operation: G.I.R.L.F.R.I.E.N.D” when she realizes Numbuh 1 holds his job higher than his love for her, she breaks up with him. While her being hurt at him being more dedicated to his job than being her boyfriend is kinda understandable, it's actually not. If she was truly his girlfriend, she would at least try to use communication to solve the problem or maybe be a better person so he would hold her in a higher priority!
She is unable to take criticism, as evident in the episode “Operation: D.A.T.E.” where she gets angry at Nigel for humiliating her in front of everyone by calling her out for being too focused on dating him, storms in to tell Nigel that it was the worse date ever and she tries to throw a cherry punch at him.
Also in “Operation: L.E.A.D.E.R.”, when both Numbuh 2 and 4 voiced their dissatisfaction with her idea of dressing up the Sector V members with superhero costumes and taking them to the beauty parlor, what does she do? She just yells at the boys to shut up, which results in all five members of Sector V bickering non-stop throughout the journey to their mission with Numbuh 3 and 5 joining in the fight against her.
She does still love Numbuh 1 (at least until “Operation: G.I.R.L.F.R.I.E.N.D”).
There are some times when she helps the KND, such as the ending of “Operation: C.A.K.E.D.-T.W.O.” where she defeats the cake monster and apologizes to Nigel for double-crossing him.
Apart from being an almost exact copy of Constance from the DCFDTL, her character design is otherwise decent.
She still has a resourceful personality, sometimes.
Had the Galactic Kids Next Door series gone ahead, much of Lizzie's behavior would have been explained as her being an alien in a human disguise.