Bianca is Finn's younger sister in The Lego Movie 2: The Second Part. She is the real-world child whose imagination creates the Systar System and drives much of the film's conflict.
Bianca first becomes important when she begins playing with Finn and his LEGO creations. Her Duplo figures are initially misunderstood as an alien invasion, leading the LEGO characters to defend Bricksburg. The apparent attack eventually becomes a story about siblings struggling to share toys, ideas, and attention.
Five years later, Bianca's creative play transforms Finn's LEGO world into a bright, imaginative realm filled with mini-dolls, glitter, and new characters such as Queen Watevra Wa'Nabi and General Sweet Mayhem. Finn sees these creations as a threat because he does not understand Bianca's perspective.
A common mix-up is confusing Bianca with a character from the LEGO worlds. She is not a LEGO minifigure living inside Bricksburg; she is Finn's younger sister in the live-action framing story. The siblings eventually reconcile and combine their creations into the new world of Syspocalypstar.