A well-developed character can have a past, or a backstory, that is revealed later in the story. The writer has thought about the character in detail, but doesn't tell the reader everything at once.
It can help to think about your character's family tree.
Draw your character's family tree and give each person a name.
Choose one family member from your family tree. Write a description to introduce the character. Are they like your character? Do they get on with you character?
Mention specific details about your character to make them seem more believable - perhaps they have:
particularly long arms?