A kindred spirit - someone I can agree with.

The term originates from the Bible (King James Version):

Samuel 13:14:

But now thy kingdom shall not continue: the LORD hath sought him a man after his own heart, and the LORD hath commanded him to be captain over his people, because thou hast not kept that which the LORD commanded thee.

Acts 13:22:

And when he had removed him, he raised up unto them David to be their king; to whom also he gave testimony, and said, I have found David the son of Jesse, a man after mine own heart, which shall fulfil all my will.