Idia explains that Grim:
1) is a dire beast with high blot density
2) is a magical fusion of a dire beast and some kind of animal
3) has a capacity for though that’s on par with humans
4) is lackluster with magic
5) potentially has phenomenally high blot accumulation, but they cannot get a precise reading
6) is resistant to all attempts at blot treatments
7) has traces of a powerful spell cast on him that may be a curse or a blessing, and involves ancient, 1,000+ year old sorcery: “it’s got layers of complex, interwoven magic…it’d take a century for (an) analysis to finish.”
Idia decides that Grim’s magic reservers make him no threat as, “even if something DID happen, it wouldn’t be super disastrous.”
While as of yet unconfirmed, the “some kind of animal” that Grim has been fused with might be a cat: he is called a cat as early as the prologue, by Riddle, who later corrects himself.
When Azul asks about Grim sharpening his claws, Riddle says that he once caught him scratching up a sofa in the Heartslabyul lounge. Azul responds, “He always insists he’s not a cat, but maybe he really is one…”
He is also called a cat by Jamil, Cater, Floyd, Ortho, Idia and Leona.
When Vil jokes about drawing Grim out of hiding in Book 6 by tempting him with tuna the prefect has the option to respond, “Come on, he’s not a cat.”
(Deuce calls Grim a cat on EN, but this is a line that does not exist in his original dialogue.)
Grim has also occasionally been called a pet, which he does not appreciate.
Throughout Book 6 a “small beast” terrorizes the various characters and is hinted at as being Grim (with what appears to be Grim’s silhouette), but it is never confirmed.
Twisted Wonderland Reference Note 