When you are coming up with characters it is always a good idea to get to know them. The more information you have on them the more realistic they will become. It also helps with the story. Their background might not come up in the book, but it helps to understand why they act a certain way. Maybe they are terrified of the dark or being alone at night because when they were young something horrible happened to them. The more information you get the more real they feel, and eventually, you will know your character more intimately than some people you know in real life.
Questions to answer about your character
What is your character's name?
How old are they?
What do they want?
What do they dream about?
Is there an experience that has affected their life dramatically? Death of a parent; Witnessing something horrific; Experiencing abuse?)
What are their fears?
What’s their favourite food?
Are they an only child?
What is their family like?
Do they have any regrets?
Physical habit? (nail biting, hair chewing, picking at their skin)
Speech mannerisms? (stammering, a lisp, an accent)
Who do they live with?
Do they have a job?
Where do they live?
What do they live in?
Who do they hate?
Do they have a favourite word?
What do they do on Saturday nights?
What is their cultural background?
Does you character have any talents?
Do they own any pets?
Where do they like to go on holidays?
How do they like to relax?
What stresses them out?
Are they ambitious?
What do they want most in life?
What is their favourite music?
Who do they most admire?
What time do they go to bed and get out of bed?
How are they feeling today?
Putting your character in different scenarios
If you answer most of these questions the next thing you can do is write a scene with your character in it and see how they react. It could be a scene where they come across a lost child, are they the type of character to help, or to harm? What about a scene where they are scared but need to think logically under pressure, how would they react in that scenario?
It is always fun to play around with scenes even if they don't have anything to do with the story. It just helps you flesh out your characters, it will also help your writing along the way.