Your costume suggestion
Black and silver robes are the obvious practical choice for a necromancer - things can get a little messy. Finish the look with long black hair and accessorize with ancient arcane symbols.
You have always lived your life in the shadow of death - your father was an undertaker and your mother smoked like a chimney. It’s no wonder then that you developed the ability to communicate with the other side. You quickly got bored of just communicating though, and began studying necromancy - the art of raising the dead. This made people fear you, yet you see nothing sinister about it and actually find it quite fun. At the very least, you’re never short of people to chat with.
INTRODUCE YOURSELF AT THE START OF THE GAME BY READING THIS
I am Icy Deadpeeps, a necromancer of great renown. I raise spirits the dead who bring me important messages and visions of the future. King Toffrey consulted me frequently to receive messages from the underworld, although I haven’t seen him since last night when he summoned me to perform a special ritual for him. Pay no attention to the blood on my clothes. It does not belong to the king.