When Usagi was a little, she was never afraid of the dark. "Moon," was the first word she learned - much to her mother's consternation and her father's amusement. Ikuko would say it took some bribing to get Usagi to call her "Mother", though her father was much more shameless in telling the tale (food being a main factor in the bribe). They never tired of the irony, calling her Usagi suddenly was not just cute but appropriate.
When Usagi was five, her mother would find her sleeping by the window. "You can't sleep there, Usagi-chan!" Ikuko would admonish her, worried she would catch a cold. Usagi wanted to listen but she couldn't help but be drawn. In the end, they compromised by moving Usagi's bed closer to the window so she could look out of it when she went to bed. Usagi always felt safer when she could see and feel the moonlight on her skin when she slept.
Sometimes, before Usagi discovered her destiny as Sailormoon, Ikuko would check on her well she slept and think of her child as a little moonbeam in the dark.