Vice Admiral Ysionris Jeromi is a UNSC naval officer and doctor considered to be a living legend.
On multiple occasions, Jeromi had taken the UNSC Hopeful, a ship with no weapons or armor to speak of, into battle to save the lives of the critically wounded aboard damaged ships. He had saved tens of thousands of lives, and more than once faced a Court Martial. He had gained and lost rank over the years, but he had also won the UNSC's Colonial Cross twice. He was a four star Admiral in 2525, and a three star Vice Admiral in 2551.
He was a personal friend to Dr. Catherine Halsey, and analyzed a number of 'hypothetical' experiments on Bonobo Chimpanzee for Dr. Halsey in 2525. She was his star pupil, and he had never approved of her working with the Office of Naval Intelligence. He was a mentor to her and his disapproval made her decisions more difficult. He sent her UNSC Priority Transmission 09872H-98, detailing his objections to a "hypothetical" augmentation process and the dangers of such a process. The experiments he conducted were later used on the SPARTAN-II soldiers. However, he was not made aware of this and likely would have objected if he had known.
In 2551, he took the SPARTAN-III soldiers aboard the Hopeful for Project CHRYSANTHEMUM. He was suspicious of Kurt-051, especially after a message arrived for the Spartan from a secret Slipspace COM launcher.