Overview
Mythology describes Cerberus as the monstrous three-headed hound guarding the Underworld, tasked by Hades to keep the dead in and the living out. In reality, he's quite intelligent, easygoing, sweet, and even a little lazy outside his primary duty. The massive influx of souls during the world's upheaval damaged the Underworld, cutting Cerberus off from Hades and flooding the afterlife with more souls than even he could shepherd. Overwhelmed, Cerberus abandoned his post to fix things before Hades decides to put all three of his heads on spikes. He can shapeshift to get around more conveniently in the mortal world, but no matter his form, his sense of smell and hearing are always superhuman. Once befriended, his loyalty is absolute. His obedience... not so much.
Perks
Guardian of the Underworld: Cerberus can communicate with the dead, whether ancient or modern.
Cerberus has the ability to speak with the souls of dead mortals, as long as the spirit is present. This holds true whether he's speaking addressing an ancient Greek hero or somebody who just went splat five minutes ago. He can sense when the dead are nearby, and track them if he's close enough. Finally, Cerberus can guide souls into the Underworld. He can always find the realm of the afterlife in his own realm, but it's less reliable in other realms, and he's not always able to find it. When he's able, he can talk to the souls there, and seek out specific souls if asked nicely. If those souls are present, anyway.
Flame Breath: All of Cerberus' forms can breathe fire, to a certain extent.
Cerberus can spit gouts of flame from his jaws no matter which physical form he assumes, although the size of his form determines how much or how effectively he can do it. Smaller forms, such as some domesticated dogs, can only breathe a tiny gout suitable for lighting a candle, igniting tinder, or just freaking people out for the surprise factor. Larger forms, such as the bigger hunting hounds, can manage a blast that can do moderate damage in combat. His true form, that of a three-headed hellhound, is capable of sustained gouts of flame hot enough to work metal with.
Keen Senses: Cerberus has heightened senses of hearing and smell.
No matter which physical form he assumes, Cerberus always has heightened senses of hearing, smell, and touch. He can listen to distant conversations or follow something by sound, as well as identify individuals by the sound of their voice or stride. He can smell keenly enough to follow a trail like a bloodhound, staying on it even through rough terrain. (As a hellhound, three noses are better than one!) When in darkness, he can feel his way fairly accurately by whisker, paw pad, or fingertips.
Shapeshift: Hellhound< Edge >: Cerberus' natural form is a three-headed dog the size of a large shed.
The most openly monstrous of Cerberus' forms is that of the hellhound. Not only was he a vicious guard dog the size of a large shed, he also had three heads that could spit gouts of flame. This is his most natural appearance, but also the least comfortable to assume: The will of the world actively pushes back against mythical beings like himself existing in the Modern Age. Assuming his true form won't kill him, but the longer he stays that way, the more sick and uncomfortable he feels and the harder it is for him to focus.
Shapeshift: Hound: Cerberus can assume the form of just about any kind of domesticated dog.
As a compromise between his monstrous form and his humanoid one, Cerberus can assume the forms of various domesticated dogs. In these forms, he has all the abilities such a dog would have, such as the nose of a bloodhound or the speed of a greyhound. He also has the same supernatural boost to his speed, strength, and resilience. This is also convenient for him, because he doesn't have to worry about metaphysical pushback from the world. Dogs are pretty normal. Even when they have glowing red eyes and coal-black pelts like something out of a nightmare. Right?
Shapeshift: Humanoid: Cerberus can assume a humanoid form to navigate human society more easily.
To better facilitate getting around human society, Cerberus can assume a more humanoid form. This lets him interact with things that are built to the scale of the human body, like "doorways" and "stairs." Though he might look a little more humanoid, his body is faster, stronger, and much more resilient than the average human. There's no mistaking him for human, though. His features almost always tend to be a little... monstrous.
Complications
Monstrous Aura: It's always obvious that Cerberus is not normal, or even monstrous.
No matter what he's doing or how his physical form might appear, Cerberus is always pretty readily identifiable as what he is. He is one of the great monsters of Greek mythology, and it's not only hard for him to hide that, it's impossible. Even in his humanoid form, it's easy to identify him as something that is very obviously inhuman. Needless to say, being around supernaturally sensitive people means that there's approximately zero hope that he's going to be able to sneak around, conceal his true nature, or otherwise obfuscate what he is in any way.
Separation Anxiety: Cerberus is a lonely doggo who doesn't like to be alone.
After spending eons guarding the Greek Underworld, Cerberus has come to the conclusion that he is a very lonely doggo and that he doesn't actually like being alone. He is at heart a social animal, and will seek company when given the opportunity to do so. He's surprisingly at ease in the company of mortals, especially the ones that aren't dead yet, and also comfortable around dogs (although dogs are rarely comfortable around HIM). If he's in a situation where he might be forcibly separated from others, he'll get distressed, so much so that he may not be able to focus on what he's supposed to be doing.
Subtle As a Dump Truck: Cerberus is objectively terrible at physical or mental subtlety.
Being what he is, Cerberus is about as stealthy as a dump truck. It doesn't matter what kind of physical form he has. He's always as stealthy as a dump truck. He's just not wired to be clever or subtle, and no amount of trying will get him there. This means he approaches situations with the same blithe straightforwardness as any mortal dog. If he sees someone he likes, he's going to greet them enthusiastically. If he sees something he doesn't like, he's going to put his hackles up and snarl at it. If other people are depending on him to be subtle, or show restraint, or use actual stealth, they're... they're gonna be real disappointed.
Such a Good Doggo: Cerberus is a little too easily motivated by praise and treats.
Even scary monsters like Cerberus like to be patted on the head and told they're a good boy. Like any self-respecting dog, his primary motivation in loyalty and obedience is the pursuit of praise, treats, and bellyrubs. Whether this means temporary loyalty to somebody in the World Tree or his unshakable loyalty to Hades and Persephone, Cerberus is motivated by the same end result. Fearsome reputation aside, he's easily bought off by praise or treats, to the point where the only thing that can't be derailed is his primary duty of guarding the Underworld. By anyone. Literally anyone. Just bring a stack of raw steaks and rub his belly. Naturally, this could have the potential to make situations very complicated, and make his allies very cross with him.