Lucy was introduced to the party as the daughter of their first benefactor, Lucius.
She was apparently saved from a group of people who had assaulted her and her ‘mother’.
Lucy was kind enough to explain to the party, while en route to Saharabellum, that shardminds aren’t born and the dynamic of mother/daughter was one which Lucius was had an easy time wrapping his mind around. She goes on to explain that as shardmind aren’t born, she isn’t even technically a child, but rather just of diminutive stature.
Lucy was the charge of the party during their travel to the site of Saharabellum. At which point, she went with some robed humanoids.