From Halopedia, the Halo wiki
A mind of ice and fire.
Serina was activated on January 7, 2530, by Daedalia Technology, a Martian firm well-known for the seeding and development of artificial intelligences for military and corporate markets. Serina was their greatest work of that era, an omni—capable second—generation "Smart Al" that displayed both impressive intellectual capacity and remarkable computational efficiency.
Her assignment to the UNSC Spirit of Fire was fortuitous, despite a somewhat rocky period of acclimation to the ship's unorthodox crew. Under the orders of Captain James Cutter, Serina administered the deployment of forces on Harvest's surface, and successfully managed later campaigns on both Arcadia and the enigmatic Forerunner facility known as Shield 0459.
Unfortunately, the life of a smart Al is limited and their constructed minds cannot be slowed or stopped. While the Spirit of Fire drifted in the void after their victory over the Covenant at Shield 0459, Serina imagined, planned, and built for a future she would never see.
Serina had a remarkable aptitude for research and analysis, and her thoughts often turned to the work of Ellen Anders. In particular, the Professor's early work on vacuum energy extraction—and its potentially useful side effects with regard to heat siphoning—fascinated Serina.
Serina's experimentation and engineering expertise gives her access to unique cryotech—enhanced units and abilities that can slow targets, and render them inert. If the chill gets intense enough the target will freeze. Frozen targets are immobile and unable to perform any actions, but don't take damage unless they are shattered. Frozen units that take a significant amount of damage or are targeted by certain special abilities [such as Seismic Charge] will shatter and immediately take a massive amount of damage. Frozen units that are left alone will thaw out after a short time, and do not suffer the damage spike.