Bella Ruffalo, born April 12, 2005, seems to be the most private out of the three siblings. She's almost 16 years old, but doesn't appear to have any social media of her own, and doesn't appear super frequently on her family's accounts either.
Her most notable public appearance so far came in 2015, when she walked the red carpet with her dad in matching tuxes (pictured here). Like her older brother, she also has had a cameo in one of her father's more famous movies: she appears in a crowd scene in Thor: Ragnarok.