Sondelius appears nearly midway through this book, and his main role seems to be that of a replacement mentor for Martin Arrowsmith after Martin's relationship with Max Gottlieb fizzles. Sondelius shares Gottlieb's commitment to science, but he's much more of an activist, travelling around the world and curing disease wherever he can. Our first impression of him comes from Martin, as the narrator notes:
In medical school, Martin had read of Sondelius, the soldier of science. He held reasonable and lengthy degrees, but he was a rich man and eccentric, and neither toiled in laboratories nor had a decent office and a home and a lacy wife. (16.4.2)
In other words, Martin respects Sondelius for avoiding many of the pitfalls of a scientific life, including the temptation to rest on your laurels or to spend your entire life locked up in a lab. In fact, Sondelius is as social as they come when he first meets Martin. Without hesitation, he takes up Martin's offer to go out for a drink, saying, "Come! I buy a drink. As a hygienist, I war on alcohol. In excessive quantities it is almost as bad as coffee or even ice cream soda. But as one who is fond of talking, I find a nice long whisky and soda a great solvent of human idiocy" (17.5.13).
Unlike many people in this book, Sondelius has nearly no ego. Sure, he's kind of a loudmouth and a bully, but he doesn't hesitate to go from being master to servant when he hears that Martin is working on some interesting stuff at the McGurk Institute. As the novel tell us, "He became Martin's collaborator; unpaid, tireless, not very skillful, valuable in his buoyancy" (30.5.8).
Sondelius is basically a workhorse of science, a great ally because he never gets tired and because he's super loyal. That's one reason that it's so difficult to watch him die of plague on the island of St. Hubert. The only reason the guy dies is because he refuses to get vaccinated against the plague until everyone else on the island has gotten vaccinated first. The guy essentially dies for his beliefs, which means he's a bonafide stand up guy.