From Halopedia, the Halo wiki
Leonard Gillespie is a scientist working for the Office of Naval Intelligence.
Gillespie began his career as a graduate student under Doctor Forester. When an old Kig-Yar point defense gauntlet was found in 2536, he managed to break and dismantle it, allowing the UNSC to learn much about the Covenant technology. Because of this impressive feat, Doctor Catherine Halsey had him transferred to Reach.