Presumably a UNSC vessel. In a promotional transmission for Halo: Combat Evolved featured on Bungie's website, the Hermes II received a message from the CAR-class Covenant Cruiser, the Contrition.
NOTE: Nothing else about this object is known. It is possible that the Hermes II is a station or even a planet. However it is referred to as a "SCS" class ship, the same letters which preceed the Pillar of Autumn. Therefore it is possible that this is a Halcyon-class cruiser. It is impossible to confirm this.
It could be noted that "Hermes" is a name that has been assigned to many ships in the British Royal Navy, 10 in fact. As the UNSC does still seem to bear a fair deal of commonality with current real world military traditions, it is possible that such old naval traditions, such as new ships reusing old names, has continued.
The Transmissions
Before Halo 1's release, a number of transmissions related to the game could be found on Bungie's website. The transmissions are directly linked to the Cortana Letters.
Below is an excerpt from the Transmissions that involved the Hermes II:
- SCS Hermes II
- RECEIVED (trans) 04.14.6.1835
- via x-process 375.2.686.0632091
- (process owner GA - AI - Class II)
Through war we will teach your heresy.
Through battle we will teach your frailty.
Through death we will teach your fate.