@Mimichan 0w0 yes. Grim's mother was a half spirit at the time of the incident Grim barely remembers. She left him outside of the asylum grounds before fully dying and becoming that full spirit form Max and Leo saw.
Half spirit of what is much more difficult to explain but my only hint is a question or a riddle I guess. "A figure tall and dark would appear in Grim's mind. Decipher what figure appears like and it'll answer his mother's kind."