Max's mother is dead (duh), but she is far from helpless. In some mythology, spirits are stronger the more love or resentment they hold as they die. (don't quote me on that I may need to check my sources) She is behind the scenes orchestrating events to test if Max is worthy of her affection. We haven't received much background info on Max's mother. Not even her name. Maybe she was a very proud, determined parent who pushed Max simply because she wanted the best for him. It might explain how he got so acrobatic, she wanted him to be a gymnast or something?
She may have been hard on Max but also nice at times, like mothers are. But maybe dying and maybe becoming a ghost have turned her bitter and as a result she believes that Max must earn her love. So secretly she is the big bad and she sent Mirror-Frog after him to test him after she carefully orchestrated the events that led him to grabbing the bat that is now his tool.
Just a thought that might be cool. Like the title says, its a 'Crackpot Theory'.