Nightmare King Grimm:

Grimm is a physical manifestation of the Nightmare Heart, a higher being that resides in the Nightmare Realm. 

Cyn:

Cyn was a major antagonist who partially served as the final antagonist in the finale of Murder Drones. She was a Worker Drone who was improperly disposed of, resulting in the Absolute Solver mutating inside of her and taking her as a host to cause destruction.

Visna:

Visna is a malevolent entity created from restless spirits who gathered to form a single monstrous being out of pure hatred. its body or rather the vessel is composed of bone fragments, teeth, and organ tissues.

Blitzo:

Blitzo has fairly broad forearms and hands, and a long narrow head, out of which extends large, curved, black and white striped horns. He has black spines on his head and upper back, as well as on his tail. His tail ends with a triangular point that has a black spot at the tip.

He has pointed feet with boot-like heels which are shown to have black on the heels and toe areas.

Blitzo wears a tattered slate gray collared coat with red buttons down the breast, black knee-high boots matching the shape of his feet, and elbow-length black fingerless gloves featuring large, rounded yellow decorations on the upper side of them. In "The Full Moon", the yellow decoration on his left glove is infused with his own Asmodean Crystal. Blitzo also wears a red skull charm around his neck.

In "Exes and Oohs", Blitzo is shown to wear boxers with horses on them.

As shown in "The Full Moon", Blitzo has red gums.

Young

In his youth it is shown that Blitzo lacked the white blotches on his body that he has in adulthood. He also lacked the black spines on his head and upper back, as well as the two stripes near the end of his tail and tail spines that he has as an adult.

As a teenager, he wore braces.

Personality

Blitzo is a bombastic, big-talking, and sardonic imp. He is high energy and career-focused, having recently started his own company, something that's noted to be rare for an imp to achieve.

He is very invested in the company's success and willing to make many personal sacrifices to achieve it. However, despite his enthusiasm to break out of the lowly social position his species is assigned in Hell. He is also, in many ways, wildly unprofessional and scattershot in his methods.

Despite his status as his company's sole director, he can frequently behave in ways that are immature, self-absorbed, and even disrespectful towards his employees, affecting their level of respect for him.

He is also willing to be manipulative in some questionable ways, sleeping with the married Stolas, initially in order to "borrow" his grimoire, the key element allowing him and his employees to fulfil contracts to assassinate people on Earth.

Underneath Blitzo's powerful drive to succeed at all costs, however, is an emotionally troubled imp, one who is dealing with the repercussions of a checkered past, that has left many people he once knew, and is implied to have cared for, hurt and actively bitter towards him. Deep down, covered by his bluster, Blitzo is haunted by these ghosts and makes a habit of running from, rather than facing, his current problems in reaction.

He struggles with genuine emotional connection, and appears to crave the idealized healthy relationship dynamics of his married employees, Moxxie and Millie, who he regularly spies on while they're in private, as well as the affection of his surly adoptive daughter, Loona.

Blitzo is slightly more at home and focused when on the ground on assassination missions, as he is murder-savvy and can be gleefully sadistic with his targets, having minimal restraint in the more brutal killings. When push comes to shove in high danger and combative situations, Blitzo is also fiercely protective of his friends and family.