Isabella

Isabella was the imaginary friend of Clara Sutter, one of the children aboard the USS Enterprise (NCC-1701-D).

When a spaceborne entity came aboard the Enterprise-D in 2368 to investigate the ship as a potential energy source for its species, it appeared in the form of Isabella and befriended Clara, claiming that she would turn invisible in the presence of grown-ups to prevent anyone else learning that she was real (Although Worf learned of Isabella's existence when the two girls ran into him in a corridor when they weren't looking where they were going). This was also an attempt to understand the nature of those living aboard the Enterprise-D.

However, the entity acquired a misunderstanding of the Enterprise-D crew, as it was learning about them from the perspective of a child. When Clara was told to stop communicating with the entity, it and others like it began to draw power from the Enterprise-D's shields, which threatened to destroy the ship. However, the entities would depart when the one that had assumed the form of Isabella was made aware of its error in judging the crew, Captain Picard clarifying that their perceived 'cruelty' to Clara by imposing rules was simply done to protect her, and even Clara will make some rules for her children when she grows up.

Before departing, however, the entity appeared one last time to Clara in the form of Isabella, expressing its hope that they would one day see each other again. 

Isabella