Well, a lot of theories in the comment points to him, but hey, I can only wait until Bleedman actually reveals who is really Mimi's parents (if it's not Him) and who really are the true parents of Minimandy (now he said Mandy isn't really her mom) and Jr.
Interesting,but at same time confusing Mandy and Nergal Jr aren't Minnie's parents? woah that's something new Grim Jr's parents are even more mysterious, theories are starting to get annoying so i guess we'll stay tuned until the truth comes to light.
Somewhere in either one of his art or comics, he hinted that Mini is from somewhere else.
Grim doesn't hate them, he just treats them differently than how a loving dad would. Probably treating them that way because he needs to keep his "Big Tough Reaper" impression and/or teaching them to be independent and tough themselves.