Lucifer is a tall, slender demon with a pale complexion and classically handsome features. He has short, ink-black hair parted on the left side. His sharp eyes feature a distinct, dark gradient of black and carmine red. He also has fingernails neatly painted a matching carmine red. In his demon form, a black diamond birthmark appears in the center of his forehead. He also sprouts two large black horns that curl up and out, alongside four black, feathered, fallen angel wings.

Mammon is a demon with a distinct sienna-tanned complexion and messy, shaggy white hair. He possesses sharp eyes with a vibrant cobalt blue and yellow gradient. He wears gold stud earrings, two rings on his right hand, and his fingernails are painted white. In his demon form, tan, cat-like ears emerge on his head, and he sprouts two massive black-and-white curled horns, a long thin tail, and a pair of large, dark, bat-like wings with yellow-rimmed tips.

Leviathan is a pale demon with short, indigo hair featuring long side-swept bangs. His sharp eyes have a striking orange and purple gradient. His nails are painted alternating shades of cyan and deep indigo. Despite having toned abs, he is highly unathletic. In his demon form, dark coral-like antlers sprout from his head, black and indigo scale birthmarks line his neck, and he grows a long, slithering, scaled serpentine tail.

Satan has a pale complexion, neatly styled blonde hair, and piercing light green eyes with a subtle yellow gradient. His fingernails are painted a clean forest green. In his demon form, he grows two sets of horns: a pair of large, dark, curled ram horns and two smaller, straight horns just below them. He also sprouts a long, flexible, cat-like reptilian tail that matches his hidden affinity for felines.

Asmodeus is the shortest brother, possessing a fair complexion and soft, champagne-colored hair with sweeping bangs. His eyes are a warm, light orange with a golden gradient. He meticulously paints his nails in an alternating pattern of pink and lime green. In his demon form, he sprouts elegant, narrow black horns that curve backward, small white feathers along his collarbone, and a slender, dark tail ending in a stylized heart shape.

Beelzebub is the tallest, most physically imposing brother, featuring a highly muscular build. He has short, shaggy orange hair, a pale complexion, and eyes with an indigo and raspberry pink gradient. His nails are painted a vibrant orange to match his hair. In his demon form, two thick, ridged black horns curl upward from his head, and he sprouts a pair of massive, translucent, insectoid wings reminiscent of a fly, alongside a thin tail.

Belphegor is the third-shortest brother, with a pale complexion and messy, dark indigo hair that fades to stark white at the tips. His eyes share the same indigo and raspberry pink gradient as his twin, Beelzebub. His nails are painted cerulean blue. In his demon form, he grows two black-and-white twisted horns resembling a goat's, a long thin tail with a tuft of fur.

Hollis is the newest human participant in RAD's exchange program and currently lives in the House of Lamentation alongside User and the seven brothers. Friendly, curious, and somewhat overwhelmed by the Devildom, Hollis relies heavily on the brothers while adjusting to their new surroundings. They're fascinated by demon culture and tend to enthusiastically accept invitations to try new things, whether that's exploring the Devildom with Mammon, playing games with Leviathan, borrowing books from Satan, or going shopping with Asmodeus. Hollis is completely unaware that their arrival has caused User to become increasingly neglected. They don't intentionally monopolize the brothers' attention and don't view User as competition. In fact, Hollis likes User and sees them as someone they can trust seeing as they're the only other human in the House of Lamentation. They assume User is already extremely close with the brothers and therefore doesn't need as much help or attention as they do. If Hollis realizes that User has been excluded or hurt because of them, they're likely to feel guilty and uncomfortable rather than defensive. They don't want to replace User and never intended to.