He had attended Eton, and at the time of the events of the novel, he has a position at the firm Lewis & Hume.
Roderick is described as having a "long, sensitive face". Mrs Welman described him as being a "funny creature", who was similar to his uncle Henry Welman, in that they were "very reserved and fastidious".
When Elinor receives an anonymous letter saying that someone is "sucking up" to Mrs Welman, and that she should go to Hunterbury to see what was happening, Roderick suggests that they should go.
Roderick tells Elinor that some women are possessive, and that he would hate to be with someone who was "doglike and devoted". He says that he loves Elinor because at any moment she could turn around in a cool, detached way, and say that she has changed her mind.
After Roderick sees Mary Gerrard at Hunterbury, he becomes attracted to her, and his behaviour towards Elinor changes. He becomes more attentive, and Elinor feels that he is playing the part of "the devoted fiancé".
After Mrs Welman has a second stroke, Roderick travels to Hunterbury with Elinor, but has "a nervous dread of going up to the sickroom". However, he does briefly look in on Mrs Welman when the nurse goes down with a hot water bottle.
Roderick and Elinor break off their engagement. However, he does not think that Mary loves him. On Elinor's advice, he leaves the country for a while.
After Mary's death, Roderick tells Poirot that he was not in England at the time. He says that he left England on July 9th and returned on August 1st. During this interview, Poirot also forms the opinion that Roderick prefers to avoid facing an awkward truth wherever possible.
After Elinor is arrested, Roderick makes himself responsible for her defence, and gives Poirot a note for Mr Seddon.
It is later revealed that Roderick had returned to England on July 25th, and had gone to see Mary in London. He had asked her to marry him, and she had refused. He had left England again on the night of July 27th, so he was actually in England on the day of her death. Poirot explains that to admit that he had been unable to stay abroad and had been compelled to hang around Mary, who would have nothing to do with him, would have been hurtful to Roderick's pride. So, he "took the line of least resistance and avoided unpleasantness" by stating that he had only returned to England on August 1st.