Combs faces five criminal counts in total, including two of sex trafficking, and has denied all the allegations.

Prosecutors are aiming to fill in the picture of the mogul’s relationship with Casandra Ventura by questioning his former assistant and Ms. Ventura’s mother.

Ventura, who is the prosecution’s star witness, testified last week that she endured physical and sexual abuse from Combs and detailed how he orchestrated drug-fueled sex performances called “Freak Of...