Mother. It was the word Arthur always used when he talked about his parents to someone else. Ironically, he could never use the word in front of the person herself.
In fact, when he first met her, no one told him that Celine was his mother. Perhaps his grandparents didn't want him to mistakenly blurt out the word and make things awkward. Thinking back, it was probably better. Arthur had no idea what his child self, who had been hearing people talking nastily about him and had just gone through a traumatic incident, would be like if he knew his mother didn't look at him like he was her child.
At least, not knowing that made him focus on his training—although said training changed his brain chemistry and added another handicap in his life. Arthur sometimes thought about it in his adulthood, and wondered if he would start throwing tantrums had he known.
Was it too late to throw some tantrums now?
