Karl is one of the Legion's frumentarii, whose duty is to scout the wastes and make contact with tribes that may be able to help further Caesar's cause. He is currently stationed at Red Rock Canyon working as an ambassador of the Legion to the Great Khans, describing his role as "a token of our alliance." While he professes to respect the Great Khans as fellow warriors, the reality is quite different, as his journal shows. Located in a footlocker in his room at the Great Khan longhouse, the journal reveals that Karl, in his time as a guest of the Great Khans, has developed considerable contempt for them and views them as nothing more than barbarous savages. He deliberately hides the fact that the Legion forbids women from being anything more than servants and slaves, even telling a female Khan that she will be able to serve in Caesar's army as an explorer.
If the Courier works to break the alliance between Caesar's Legion and the Great Khans, Karl must be disgraced in the eyes of the Khans, most importantly Papa Khan. He can be verbally taunted, tricking him into angrily blurting out a grave insult to the Khans in front of the longhouse. Alternatively, Karl's journal can be stolen from his room and shown to Papa Khan, who looks at it in disbelief before exploding into anger. Either way, the Khans attack Karl and kill him at Papa Khan's command.
If the Courier aims to preserve the alliance between the Legion and the Khans, on the other hand, Karl survives and completes his mission as Caesar's ambassador to the Great Khans.
If the Courier kills Karl, the Khans will not become hostile. They will, however, receive infamy with Caesar's Legion. It does not seem possible to discredit him with either Regis or Papa Khan if you do so.
Interestingly, Karl pronounces the word Caesar with a hard C sound sometimes when approached (Kai-sar) but pronounces it with a soft C (Seize-er) in most of his text. This is most likely an error of taking a generic legion text file, and applying it to a unique non-player character of the same voice actor. His English pronunciation of the word is more like everybody else who isn't affiliated with the Legion, despite being a member of it. The same glitch/quirk can be heard in the dialogue file of the Legionary guarding the weather station at Fortification Hill, who is voiced by the same actor.
Karl also seems to have an honorable view of the Burned Man while most members of the Legion despise him.
If the Courier has a positive reputation with the Legion, Karl will remark on this and recognize the Courier upon first interaction.
Likewise, even if the player character is vilified with the Legion, he won't be hostile on sight but still say that the Courier is perilously close to making an enemy of Caesar, even if Caesar is dead.
If the Courier kills Caesar, Karl will angrily threaten him/her, saying that he would avenge Caesar if they weren't on Great Khan soil. In some cases, he may attack on sight, but the Great Khans won't intervene for the Courier or Karl.
Karl will remain at Red Rock Canyon even if the Courier has already killed all the Great Khan inhabitants.
Halfway through the quest Oh My Papa, Legionary assassins will attack the Courier saying that "Karl sends his regards". If they are defeated, Karl himself will appear a few days later and attack the player character after initiating dialogue.
If Karl is killed, his footlocker in his room (inside the longhouse) can be looted freely with no loss of Karma.
Karl appears to be a vegetarian because unlike everyone else in the Longhouse there is only fruit on his plate instead of meat. Also, even though alcohol is supposedly forbidden under Legion rules, his beverage is a bottle of Wine.