Arthur is a wealthy English child living with his mother, Mrs. Milligan, on the houseboat *Swan* during Rémi's travels through France. He suffers from a severe, unspecified illness that confines him to bed for long periods, leaving him frail and isolated. His health limitations and restricted mobility deny him ordinary childhood interactions, making him a lonely figure.

Arthur first meets Rémi when Vitalis's troupe performs on the *Swan*. Despite his privilege, Arthur shows no arrogance. He is gentle, curious, and emotionally open, forming an immediate bond with Rémi. He is drawn to Rémi's warmth and musical talent. Rémi's companionship provides Arthur vital emotional support, alleviating his isolation. Mrs. Milligan actively encourages this friendship, recognizing its benefit to her son.

Arthur's father died before his birth. His uncle, James Milligan, orchestrated Rémi's abduction as an infant to eliminate him as an heir, ensuring James would inherit the family fortune should Arthur die childless. This inheritance scheme forms the core conflict separating Arthur and Rémi in infancy. Arthur's survival despite his grave illness repeatedly defied medical expectations.

Arthur's health significantly improves after Rémi rejoins the Milligan family. He regains the ability to walk, symbolizing both physical and emotional healing following the family reunification. This recovery coincides with the exposure of James Milligan's schemes and the resolution of the inheritance dispute. In the conclusion, Arthur is formally adopted into the Milligan household alongside Rémi, solidifying their fraternal bond. His restored mobility and integration into a stable family unit mark his definitive development from a vulnerable invalid to a hopeful, active child.