@Johan - I can see where you are coming from. Dieter is far from an arrogant aristocrat. Not so much immoral as amoral, he stands as an inquisitive, apathetic observer most of the time, assessing people and situations while throwing out morality and the concept of gray areas as contaminating factors. Driven by what we would call a strong streak of social Darwinism, Dieter takes great pleasure in determining whether the will and/or strength of one person or party is capable of overpowering those of another and genuinely respects people of strong will and unshakable conviction, but cares little to nothing for human lives that are affected or even lost as a consequence of the conflicts that arise.
The Church and State of Talingarde is the opposite of this. They take the sweat from a man's brow and give it to the most undeserving and mewling. They promote inefficiency and call it charity. By supporting the poor, they victimize the successful. To Dieter, Asmodeus is more moral than Mitra - Asmodeus demands tribute, but Mitra steals from you with a smile and teaches you are bad if you disagree.
@Heinrich - Reading through the gameplay thread, it appears that the Ninth Knot is still trying to break the silver seal binding the plague demon. The Cardinal mentions that previous knots have failed in this task, which means it is likely that other knots operating in Talingarde are experiencing casualties on dangerous missions. Since the Ninth Knot is now in a position where they will need additional assistance... perhaps Dieter is already a member of another knot. He has successfully served Cardinal Thorn for several months, but the latest mission his knot was assigned proved disastrous. Several members were killed, leaving them with partial strength and utility. Cardinal Thorn decides to dissolve the knot and shuffle the members into other knots. Dieter is assigned to join the Ninth Knot and assist them in their task. Why Dieter? Because Dieter is a very capable manager. His goal will be to help the Ninth Knot increase community penetration, recruitment of agents, and acquisition of mission-critical resources. He is also a team player, and while devoted to the cause of Hell, helps bring stability and peace to the teams he works with.
Personality wise, there will be some good dramatic tension with the party. As I mentioned, Dieter is not immoral, just amoral. He will likely disapprove of unrestrained violence and slaughter. Why? Because you are killing prospective subjects and able-bodied workers. That is why. Dead men do not bake bread or shingle roofs. He will likely want to incorporate contingencies upon contingencies and disagree with more hot-blooded members. A quick decision is typically a bad decision. Intuition is another word for 'making things up and hoping for the best.'
I can see Heinrich and Dieter butting heads, since they will be tied for the highest Intelligence scores in the party. They are both used to being the smartest people in the room. Luckily, their very different skill sets should be enough to set them apart, and hopefully they develop a relationship similar to friendly academic rivals.
For the rest of the party, Dieter will strive to be the quiet word whispered in the ear. Pointing out potential problems, helping to plan, stepping in to humbly assist when his skills are needed. He is not interested in ruling. He wants to be Hell's steward.
TLDR: Mr. House from Fallout New Vegas mixed with an evil executive assistant.