An innocent person is someone who won't do anything wrong. Someone who won't cheat on you. Someone who won't steal your car keys if you drop them on the floor. Someone who won't degrade others to feel good about oneself.
The epitome of innocence is the human infant. Infants smile frequently, thus making them appear more friendly. They have big round eyes, making them look innocent. And they always cry when another insults them, blames them, or accuses them of a wrong.
A lot of people act innocent, when they're actually not. For example, they will behave "nice" to others but then will gossip behind their backs. This is because they don't have the guts to be direct and honest. Innocence, therefore, may be a cowardice.
Also, many people won't break any rules, not because they are good people, but don't have the guts to risk getting in trouble.