I’m General Mayhem. Bring me your fiercest leader.
~ Sweet Mayhem
You don’t understand! We’re the good guys! The Duplo invaders wanted to play with you, but they were afraid 'cause you kept attacking them. The Queen sang an entire song on how not evil she was.
That was the TRUTH?! You people are terrible communicators.
I know. This ceremony won’t cause the “Armamageddon”. It will just unite two worlds.
~ Sweet Mayhem and Lucy; as the former reveals the truth about the Systar system to the latter
Sweet Mayhem is the secondary tritagonist and the (false) secondary antagonist of the 2019 film The LEGO Movie 2: The Second Part. She is a minidoll from the Systar System, who captures all of Emmet's friends to take them to Queen Watevra Wa’Nabi for seemingly sinister reasons, but it was revealed that she and her boss intend to bring peace between the Systar System and Apocalypseburg.
She comes first as an intruder by chasing Emmet and his friends in Apocalypsburg. Due to the small size of her ship, she can only take 5 people. As she scans Emmet's personality, she defines him as weak and not tough. Even with Lucy defending that he is The Special, she takes Lucy and the others in her ship and they go to the Systar System.
Once they come to the Systar Sistem, they met with Queen Watevra Wa'nabi where she convinces everyone to join to her wedding except Lucy, who knows something's up with her. When they take them to a spa, Lucy accidently reveals her own original hair color. Mayhem takes Lucy to an extra treatment to change her by inserting the song "Catchy Song" to change her, but she escapes.
After a while, Mayhem was doing her duties for the wedding as she suddenly attacks Lucy and they fight. However, when Lucy grabs a explosive heart, Mayhem almost falls making her helmet drop off and revealing her own face. She asks Lucy to help her, but Lucy refuses after all she did. Using her sad lament face, Mayhem falls but Lucy rescues her after she was convinced. Lucy starts talking all about the plan and what happened, but Mayhem reveals the Systar System was never meant to be antagonistic to Apocalypseburg, but instead were trying to establish peace between them, and simply failed at communicating this well. And all of that started when Emmet made that heart to the aliens which is actually Wanabi. Mayhem's tough persona was all in her helmet, because she wanted to act like the Apocalypsburgens, even if she realizes it wasn't a good idea. Lucy also discovers her friends are not brainwashed, but rather enjoying the Systar System. She thinks everything will be fine now they're at peace until she realizes Emmet is about to break the cake.
This creates a hole in the LEGO universe, and in the real world, they're all taken to the storage known as "Armamageddon". Thinking it is all over, Mayhem since there's nothing they can't do except singing a song. This somehow works, making Finn free them for all and they start to build spaceships as they get ready to battle Rex's forces. Mayhem helps Lucy to get to the Dryer System even though she doesn't know who Rex is.
After Rex vanishes out of existence, Wa'Nabi and Batman finally wed, represented by Finn and Bianca reconciling with each other and agreeing to play together again. Their mother returns their toys, averting Armamageddon. The LEGO universe is transformed into a mishmash of Apocalypseburg and the Systar System called "Systocalypstar" which is peaceful. Mayhem is last seen with Benny sharing their helmets and sightseeing the beautiful sunset with Emmet and others.
Sweet Mayhem is similar to Enfys Nest as they both initially act like villains until they revealing their good intentions.
She appeared as the secondary antagonist at the beginning of the movie, but later revealed her heroic intentions, stating that the acts she had committed were not bad.