Alzen was a Srivani researcher who secretly conducted experiments on Voyager's crew while hidden out of phase aboard the ship.
Alzen was a scientist of the Srivani, a species that studied other beings by conducting experiments on them without their knowledge. Using technology that kept them out of phase with normal matter, she and her colleagues moved invisibly among Voyager's crew, inducing illnesses and strange symptoms to observe the results.
In Scientific Method, Seven of Nine's Borg implants allowed her to perceive the intruders. Janeway, herself suffering from the experiments, flew Voyager dangerously close to a binary pulsar, gambling that the Srivani would not risk their own lives. The gamble worked, and Alzen and her team abandoned the experiment.