Considering that this man is the father of David's first wife, we don't actually get to see very much of him in the novel. Mr. Spenlow is David's immediate boss in the proctor form of Spenlow and Jorkins. He is a hard-headed businessman who runs their office skillfully. Mr. Spenlow invites David out to his house for a dinner party one evening, which is how David finally meets the lovely Miss Dora Spenlow.
Mr. Spenlow is an extremely protective father. We realize the level of his protection of Dora because (a) she's incredibly spoiled; (b) he hires Miss Murdstone to be Dora's daily companion and chaperone (ick!); and (c) when he finds out that David has been courting Dora on the side, he flips. David pleads with Mr. Spenlow to allow him to keep seeing Dora, but Mr. Spenlow replies tellingly: "Pray don't tell me to my face that you love my daughter, Mr. Copperfield" (38.39).
Dora's father is so protective that he insists that David not even admit that he loves Dora – as though David's love itself could be harmful. Mr. Spenlow also threatens that he will not allow Dora to inherit his fortune if she continues to see David. Mr. Spenlow, while he is considerably less damaging than Mrs. Steerforth, is as jealous of his daughter's love as Mrs. Steerforth is of her son's.
It's funny that Mr. Spenlow should say that thing about inheritance because that very night, he dies. And, in an extraordinary move for a lawyer who deals specifically with contracts and wills, he doesn't actually leave a will. What they do find is that Mr. Spenlow isn't as wealthy as everyone had thought.
Dora goes to live with her aunts, but she needs someone to step up and protect her in the manner to which she has been accustomed. And that someone is David. Mr. Spenlow raises Dora to be so dependent and childish that Dora needs a man in her life to be her daddy. Mr. Spenlow's sudden death leads exactly the marriage that he spent the last day of his life trying to prevent.