Most of her childhood had been spent going from one starship to another, following her father on his different assignments. Often, just as she'd make friends, her father would be transferred to another starship. To cope with this, she created an imaginary friend named Isabella; although her father was briefly concerned about this, Counselor Troi assured him that it was a perfectly normal activity.
However, shortly thereafter an entity boarded the Enterprise-D in search of a potential energy source. It took the form of Isabella and befriended Clara to investigate the ship, turning invisible when Clara was in public to preserve the idea that she wasn't 'real' (Although Worf saw Isabella when the two girls ran into him in a corridor when they weren't paying attention). Although Isabella was initially willing to lead her species in destroying the ship due to the adults' perceived 'cruelty' towards Clara, she was convinced to depart when Captain Picard convinced Isabella that the rules they imposed were only there for Clara's protection, and even Clara will make some rules for her children when she grew up. The entity ultimately realized its negative effect on Clara and left the ship.