This would be Dimmesdale and the argument takes place in Chapter Eight, "The Elf Child and the Minister."  Governor Bellingham (who is based on a real person, btw) wants to take Pearl away from her mother, Hester, doubting that she is morally fit to raise the child. Dimmesdale's defense of Hester and Pearl stems from his own guilt (he is the father) and his reasoning is that God made a decision to give Pearl to Hester as a moral compass, guiding her back to faith.  What God has done, he argues, should not be questioned.  

Here is a part of Dimmesdale's reasoning:

"God gave her the child, and gave her too, an instinctive knowledge of its nature and requirements, -- both seemingly so peculiar, -- which no other mortal being can possess.  And, moreover, is there not a quality of awful sacredness in the relation between this mother and this child?" 

Continues Dimmesdale:  "For if we deem it otherwise, do we not thereby say that the Heavenly Father, the Creator of all flesh, hath lightly recognized a deed of sin, and made of no account the distinction between unhallowed lust and holy love?  This child of its father's guilt and its mother's shame hath come from the hand of God, to work in many ways upon her heart...It was meant for a blessing, for the one blessing of her life!"