In the novel They do it with Mirrors, Christian Gulbrandsen was the son of Eric Gulbrandsen, the first husband of Carrie Louise Serrocold. He was Eric's oldest son from his first marriage, and so was a stepson to Carrie Louise, although he was actually two years older than her. He was the half brother of Mildred Strete. He was one of the trustees of the Gulbrandsen Trust.
Christian is described as "a big heavy-featured man, with a slow methodical way of talking", and his eyes are small and shrewd. There was a strong resemblance between him and Mildred. The age difference between them was nearly thirty years, so they might easily have been mistaken for father and daughter.
Christian mentioned to Miss Marple that Carrie Louise was an idealist, and that she might not appreciate the evil there is in the world. He also asked about Carrie Louise's health, in particular, her heart.
On the day after his arrival at Stonygates, Christian went out to meet Lewis Serrocold as he was returning to the house through the park. They paced to and fro up and down the terrace, and Miss Marple noticed that they both looked disturbed. Miss Marple overheard scraps of their conversation, including "how to spare Carrie Louise the knowledge", and a suggestion that they "take outside advice".
After dinner that evening, Christian excused himself, saying that he had an important letter to write. He was later found dead in his room. He had been sitting at the typewriter, and had been shot in the head. Carrie Louise said that he would not commit suicide, and was such a competent person that it could not have been an accident.
It is revealed that Christian had begun to suspect Lewis Serrocold of embezzling the Trust funds. He said nothing at the time, as he was the kind of person who would take time to satisfy himself that his suspicions were either justified or unfounded before acting on them. Carrie Louise described him as slow and painstaking, but actually very shrewd. Once he had confirmed his suspicions, he came to Stonygates to confront Lewis.
In Murder with Mirrors (1985), Christian is played by John Woodvine. His name is changed slightly to Gilbranson. He is not Swedish and has an office in London instead. Like in the novel, he is on the Board of Trustees for the Gilbranson foundation. He knows Jane Marple personally, and since Ruth Van Rydock does not appear in the show, it is Gilbranson himself who asks Miss Marple to visit Carrie Louise at Stoneygates.
In Jeux de glaces, the TV film adaptation of Agatha Christie's They do it with Mirrors by France Télévisions, the parallel character is named "Pierre Montauban". He is the best friend of the late first husband of Rose-Marie Bousquet. After the death of her first husband, Pierre helped Rose-Marie manage the fortune left to her by her husband, a large part of which went to help Rose-Marie's second husband, Etienne Bousquet set up a rehabilitation centre for youth who had committed violent crimes. Pierre observed some irregularities in the accounts of the centre which was named "La Main Tendue" (the Outreached Hand). He came to the centre to further his investigations but was murdered shortly after arrival. The manner of his killing is similar to that portrayed in the original novel. Montauban is also the owner of the newspaper "La Voix du Nord", the newspaper that journalist Alice Avril works for. The character of Pierre Montauban is played by Gérard Pinteau.