"I say, though there is not that tie between us, do not, I entreat, assail me with unnecessary taunts, or misinterpret what I say, or would say. I was only going to suggest to you that it would be a mistake to suppose that it is only you, who have been selected here, above all others, for advancement, confidence and distinction (selected, in the beginning, I know, for your great ability and trustfulness), and who communicate more freely with Mr. Dombey than any one, and stand, it may be said, on equal terms with him, and have been favoured and enriched by him--that it would be a mistake to suppose that it is only you who are tender of his welfare and reputation. There is no one in the House, from yourself down to the lowest, I sincerely believe, who does not participate in that feeling."

The speaker is an employee with history of having been guilty of a theft. He is talking to his brother, who hates him. The brother is the Manager of the firm's office. The Manager is devoid of "trustfulness"; he is very vigilant, and earned his high position by his "trustworthiness".

I think Dickens made a poor word choice.If I am trustful, I trust you. If I am trustworthy, you trust me.