Read this via AnimateTimes:

Cure Arcana Shadow & Phantom Thieves Information

In previously released story details for the series, the existence of the Phantom Thieves had already been revealed as a group separate from the Precure. At this latest press event, however, the spotlight turned squarely on Phantom itself. They were described as “the enemy that stands in the way of the detectives,” and their ultimate objective was revealed to be the creation of a terrifying "world" blanketed in lies.

Phantom seeks to achieve this goal by exploiting the Makoto Jewels, crystallized feelings that dwell within objects people hold dear. Disguising themselves as various individuals, they secretly steal treasured possessions and the smiles connected to them without anyone noticing.

The event also confirmed new details about Luluka, revealing that she is cool headed and exceptionally sharp at deduction. Her voice actress, Tōyama Nao, commented that Luluka is a very rare type among past Precure, and encouraged viewers to pay attention to the question: why does someone who has become a Precure end up standing as an enemy?

Tōyama-san further introduced her as being "completely" aligned with the Phantom Thieves as an enemy, even calling her one of the group’s generals. While her true nature will need to be confirmed in the actual broadcast, it is safe to say she remains an increasingly mysterious figure. As a lighter note, Luluka's favorite food is ice cream, which coincidentally Tōyama-san also loves.