Mr. Brownlow, a good friend of Mr. Grimwig, calls him "a worthy creature at bottom." Mr. Grimwig is, however, quite eccentric. He dresses oddly, worries about slipping on orange peels thrown on the sidewalk and killing himself, and likes to say “I’ll eat my head, sir” if what he asserts proves to be wrong.
Although Mr. Grimwig suspects that Oliver Twist is a fine young person underneath, he still feels compelled to play the devil's advocate about him. He doesn't want Mr. Brownlaw to be too quick to trust any stories Oliver tells. Mr. Grimwig wants proof that Oliver is the good person he says he is and predicts, after Brownlaw gives Oliver new clothes, books, and five pounds, that Oliver will run back to his thieving friends.
Oliver does not do this, but Mr. Grimwig's suspicions and desire for evidence work in Oliver's favor. Grimwig is a former lawyer and is able to find and bring in witnesses that show how Oliver's connection to a wealthy father and a fortune have been maliciously hidden from him. This implicates the Bumbles, who end up themselves in the workhouse where Mr. Bumble for so long oppressed others.
By the novel's close, the kind-hearted Mr. Grimwig accepts Oliver and becomes a friend to him. He is able to laugh at himself for formerly distrusting the boy.