An energy life form from a nebula that took the shape of Clara Sutter's imaginary friend to study the crew of the Enterprise.
Isabella is the name of Clara Sutter's imaginary friend, a role taken over by an energy being from the nebula the Enterprise is exploring. Appearing as a girl, she becomes visible to Clara and hides from the adults.
In Imaginary Friend, Isabella judges the crew by the way they treat Clara, regarding them as cruel for imposing rules on her. She threatens the ship and attacks Troi before Picard explains how parents care for children. Satisfied, Isabella leaves with her people.