From Halopedia, the Halo wiki
This unidentified UNSC Navy captain was stationed on Reach during August, 2552. He, at the request of Dr. Catherine Halsey, upgraded John-117's neural interface to become compatible with Cortana.
When John first saw him devoid of an insignia, he became uncomfortable, but relaxed after the captain identified himself. He was most likely killed when the Covenant attacked and glassed Reach the next day.