Innocent and simple are the words we generally use to describe a person who is plain, humble and harmless.

Credulous and gullible are two words which can be put to the same category.

Credulous( kra-du-lus ) means something apt to believe or someone too ready to believe things.
Gullible( ga-li-bal ) is used to describe a person who can easily be deceived or cheated.

The boss works in such a credulous manner that rarely anyone questions his intentions.

His credulous and gullible nature lands him into trouble on many occasions.