Rolf Muller was a human criminal.
Because of the quarantine of Earth after the attack on Earth by the Drakh, Cynthia Allen's business dried up. She was unable to get a loan to keep her business afloat, so she went to Muller to get some money. She took 50,000 credits from Rolf Muller with 100% interest compounded every 6 months. After she made the deal with Muller, she was followed by him wherever she went.
During the month of July in 2267, Cynthia Allen and Max agreed to meet each other on Babylon 5. Max agreed to pay the initial 50,000 credits back to Muller. After giving Muller 50,000 credits, Cynthia's cat was kidnapped and an ultimatum was given to repay the additional 50,000 interest. Max eventually threatened Muller and used alien technology he recovered from a dig to blackmail him, and the debt was settled.