Nils Rykermann was a biologist, resistance fighter, and politician during his more than 150 years on Wunderland. A colleague and close companion of Dimity Carmody, he served as chief biologist at the University of Munchen. In the years prior to the Kzin invasion he was well respected in academic circles (although he later admitted he was always intimidated and awed by Dimity’s intellect). After the first Kzin landed on Wunderland, Rykermann discovered that news of previous attacks such as the Angel's Pencil incident, as well as others within the Serpent's Swarm itself, had been deliberately suppressed by the ruling aristocracy of Wunderland. Feeling it his duty, Nils took up arms against the invading Kzin. His resolve was further strengthened upon the apparent death of Dimity Carmody at the hands of the Kzin fleet, when the slowboat she was escaping on came under fire and was heavily damaged before it made it out of the system. A firsthand account of the invasion of Wunderland is told by Rykerman in the story "One War for Wunderland." Rykermann would later go on to marry another former student of Munchen University by the name of Leonie. Although he never told her about the love he felt for Dimity, Leonie secretly knew and rather than hold it against her husband, she chose to never mention it as well.
In 2408 C.E., after doing battle with a detachment of Kzin, Nils and Leonie found themselves trapped in one of Wunderland's myriad caves, and surrounded by Morlocks. In order to survive the ordeal, the two humans struck up an alliance with the only surviving Kzintosh in the cave, an as yet unnamed and recently promoted Sergeant who would live to become the legendary Raargh-Hero. The three warriors were able to fight off the Morlock onslaught for a considerable time, although all of them were badly injured in the process. Due to having saved each other's lives, a sort of grudging friendship was established between the humans and the grizzled Hero. Due to this respect the Sergeant helped Leonie make her escape from the cave system, although she nearly lost her life in the process.
Later on more Heroes arrived and rescued the Sergeant, taking a gravely wounded Nils Rykermann captive in the process. Due to the bravery the humans had exhibited Nils had his wounds tended to, and as he was apparently dying was granted Fighter's Privileges, a form of last request typically reserved for mortally wounded Heroes. This request was used to formally grant the Sergeant with the partial name of Raargh-Sergeant.
Nils Rykermann somehow survived the wounds he had sustained fighting in the caves. After the war ended he entered politics and exerted a large influence over human affairs on Wunderland for sometime. For some years, he harbored secret ambitions to exterminate all remaining Kzin on Wunderland,and as such he was susceptible to the influence of other plotting Exterminationists, such as Jocelyn van der Stratt. Thankfully, Leonie harbored no such ill-will towards the former conquerors, and she along with Raargh-Hero, were able to persuade Nils otherwise. Soon after, he helped them, along with Vaemar Ritt and Arthur Guthlac, to defeat the rogue Kzin led by Henrietta at Chuut-Riit's secret hideout.