Lol They'll be about the Age Sebastian is in the Anime right now, maybe a couple years younger. Also what he looks like is up to you though I would make him look like Sebastian if possible, and I'm thinking they looks like teenagers to young adults. Here's an avatar creator that I use if you want to make him.
YAY! Okay so Do you mind playing Sebastian? I have an idea where he had a pupil in Hell. Now she's come into this world to find him, as well as being their next door neighbor and daughter to a very famous fashion designer that Lizzy gets all her gowns from. She's about Sebastian's age in our world, but in Hell she is still considered a child until she finds a mate. Which is where my idea comes in. You see, she is a cat demon who has a major love crush on Sebastian. So at night she's been sneaking out in her demon form and eating the souls of those who are destined to die and go to hell. Though being a beast type of demon, she can easily get away from the reapers by turning into a cat in a puff of smoke before they even get there. That's what I have so far, feel free to add what you'd like.