Belle was the holographic daughter in the Doctor's family program, whose death taught him about grief.
Belle was the youngest member of the holographic family the Doctor created. In the revised version of the program, she was a spirited girl who wanted to play the dangerous sport of Parrises Squares despite her father's worries.
In Real Life, Belle was badly injured in an accident and died, and the Doctor was so pained that he wanted to end the program. Tom Paris urged him to see it through, telling him that facing loss was part of the human experience he wanted to understand. The Doctor returned and stayed with his family through Belle's final moments.