Firstly you need to give your character identity. Who are they? What is their name? Where are they from? But more than that they need a history, even if it is not explored in the story. As the writer you need to know, for example, who are their parents? What school did they go to? Did something occur in their past that effects their everyday life today. In order to tell the present or indeed the future, you have to know what happened in the past.
Also as has been mentioned in a previous comment, your character needs a personality, something that distinguishes them personally. You may think you know all there is to know about your character, why wouldn't you, you made him up right? Wrong! You will find your character has always existed, whether that's in your head or not, you will find your character has certain traits and characteristics you never even dreamed of. It is your job to explore that character, don't treat them as a figment of your imagination, treat them as a person in their own right, meet them and find out all you can about them.
There are different writing exercises that are brilliant for characterisation that can really enhance your vision of your character. I advise trying to find some online and see for yourself what you can discover about your character that you didn't know before. I have used such techniques in my writing and where I thought my character was a good and just person, I soon learned they had a dark side, an almost sinister inner self. No matter how big or small the discovery, whether you realise they were a smoker or an alcoholic, it is something new that you did not know before. You need to know all there is to know about your character and it takes more than just a good imagination.
