Karl is a man first met in Ruan.
He's a senior orbal engineer at the Central Factory and is usually found working in the workshop on the factory's 3rd floor.
Estelle and Joshua first run into Karl when he's resting up in the Aqua Rossa Bar in the southern part of Ruan. Karl's current projects at work involve orbal firearms and he had a visit to Ruan's weapon shop planned. He first planned to take an airship to Ruan, but since air traffic was still on hold due to the sky bandit incident, Karl had no other choice but to walk all the way from Zeiss to Ruan.
The trip on foot gave him more trouble than just sore feet as he was chased by a monster while traversing the Aurian Causeway. To add insult to injury, he lost an important orbal gun prototype he was carrying with him during the attack, which he didn't realize until the day afterwards. He put in a request with the Ruanian Bracer Guild and was eventually given his prototype back which Estelle, Joshua and Kloe found after battling a group of Helmet Crabs near the Sapphirl Tower, which took the weapon because they were attracted by the septium inside. Karl acts overjoyed upon getting his prototype back, offering hugs and kisses to all three of his benefactors. When his offer is turned down, he gives them an attack2 quartz instead and heads back to Zeiss soon afterwards.
Karl is met again when Estelle and Joshua reach the Central Factory in Zeiss where he greets them upon recognizing them. Karl spends most of Estelle's and Joshua's stay in Zeiss working on his firearms project, which takes a blow when his colleague Hugo approaches him and asks his opinion on his assistent Louise being transferred to the Arseille engine project, which has been suffering setbacks due to a shortage of manpower. While Karl and Louise have very different opinions about design philosopy, Karl nevertheless acknowledges her capacities and despite the fact that he'd be sad to see her go, he nevertheless approves the transfer, reasoning that Louise has lost her drive working on firearms and has always had an interest in orbal engines, so she'd be much more likely to live up to her potential there.
When Louise was still working with him, she and Karl frequently had disagreements on whether to focus on performance or accessibility with Karl never missing a beat in emphasizing performance. He's forced to reconsider his views when dealing with Radmira, a customer visiting Zeiss with barely any knowledge of orbal technology, who demands a product that's easy for her to use. Remembering that Louise and weapon shop owner Stain held similar views, he decides to start looking into the matter for the first time in his career. Acknowledging that this is uncharted territory for him, he decides to write a thesis on user accessibility in order to get a clearer picture for himself.
Karl is an extremely capable engineer who has designed a wide variety of products ranging from orbal cameras to orbal firearms. However, his mind is that of an almost stereotypical technician who only judges orbments by their efficiency and who focusses on raw energy output and technical specs at the expense of everything else, even if the push to maximize energy output results in a product that handles poorly. When Radmira keeps talking about user-friendliness, Karl is merely left confused since user-friendliness is an abstract concept that cannot be gauged by measuring equipment. He's still adaptable enough to start reconsidering his design philosophy though and around the time Estelle and Joshua head to Grancel, Karl is starting the process of writing a thesis about user accessibility.
Karl is one of the people who isn't kept in the loop about Professor Russell getting kidnapped and subsequently rescued. While he notices Russell's absence in the workshop, he assumes the professor simply went home early.
Karl is the superior of Louise until she transfers to Hugo's project
He's also Elkan's main contact with the factory's orbal weapon division.