Grimm is shrouded in mystery for most of the plot. He is secretly the son of a weapon that once affiliated with Death's Eight Guardians. Being alone for so long caused him to gain a high level of Madness, but over time he learns to tune his Madness Wavelength finely and have control over it and its power.

Because of high amounts of Madness, Grimm's personality tends to show a lack of sanity. This dominant trait shows most in his sinister grin that he is almost always carrying. After joining DWMA, he fights his enemies with a killing intent. His fighting style is wild and hardly coordinated, along with most everything he does. He is extremely unpredictable. He enjoys fighting, so in most cases, he will toy with his opponents, showing his arrogance and overconfidence. (In his first encounter with DWMA, he does not reveal his intentions are pure until late in the fight when he is close to winning.) He has absolutely no fear, even showing no shame around women. He disregards any consequences that may come with what he says, therefore usually he speaks without thinking.

(I tried uploading a close up of his face, but for whatever reason, it won't save. So apologies for the preview, until I can get a more recent drawing done.