If you want a vampire, werewolf, or any thing your mind can imagine.
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Decide if it is to be open or if you wish it to join a family
If you want it in a group or family you can do this two ways, one create the character and let it stumble in on it's own. But you might be surprised if it goes a completely different direction. The second way is to talk to the leader of that group or family and ask if you can add to them by creating a new character. That has a storyline connection to them in advance. They maybe same blood line, distant relative etc. Do not create a character that is someone ones long lost brother or lover that they never heard of and just worm you way into their family because I would call you an imposter and kill you my self.
Decide it sex, origin and mental state.
Example: Male from Vampire from Scotland and is 1200 years old. Ok now, I take mine a bit further then most I go look at the time line for Scotland 1200 years ago and see what happened then, what my character would have been like before they were made into a vampire. Then look at what happened to the places he lived in from then to now to see what kind of personality he would have and what his linage would be. We are nothing with out our memories and histories. Nor are your characters, they have very little substance unless you give it to them once you do they will create their own stories.
Write a Detailed History
I will never complain if your history is too long, maybe if it is too boring but never for too long. I know that is you and yours it should be a short story of his life. Let him or her tell it on the page. I find it funny that people can have a 1500 year old character and their history fits in two paragraphs.
Write out what he/she is like now.
Physical description, mental description, based on their history and what it is like now.
Spells and abilities
I know for many this is the hardest part. But if you base it on your past, your blood your history type of character it will get easier. If you creature is an ice man, his spells would be cold, water, etc things befitting him to help him survive. Also be very specific. If you leave very broad and unclear brush strokes, your spells will not be accepted or made very hard to use. For example can call any wind to do any thing. You may think that means you can make a wind to cool off or push someone away. I look at it and see you create a wind that will cause the worlds largest title wave. That is why there are levels to each spell
Level 1 can blow out a candle make a nice breeze to cool with power of fan
Level 2 Can create a wind that will around you that can move objects under 30 lbs
Level 3 Can make a wind funnel of 150 mile an hour wind that if crossed would toss and push most away.
Level 4 Can cause a direct stream of air pushing 180 miles an hour at target throwing them far away.
Level 5 Can Now can make a wall of air 10 feet high by 10 feet wide all swirling to protect anyone behind it.
IF you break them down like such you have a much better chance of getting it approved.
Have a great and fun Roleplay