Gina Cross and Miller are responsible for the training of scientists and security guards, respectively. When their work requires them to be elsewhere in the facility, they are replaced by their holographic counterparts, to which they gave their appearance and for which they recorded the instructions. While Richard Keller, Isaac Kleiner and Colette Green also supervise Hazard Course trainings, they don't seem to have holographic counterparts. Additional scientists often monitor the trainings from nearby observation rooms.
Used in the Black Mesa's Hazard Course, the Assistant directs scientists and security guards to the different hazards they might meet during their work at Black Mesa, while it is monitored by two scientists in a lobby above both Hazard Courses. Scientists are above all trained to the use of the HEV Suit during the standard Hazard Course, while security guards go through less dangerous tests, since they are only protected by their simple helmets and Kevlar vests, during the Black Mesa Hazard Course for Security Guard Training.
An Holographic Assistant will sometimes appear within a two-part projector (its two parts being above and under), but often without any projector visible.
In the Opposing Force chapter We Are Not Alone, Shephard is allowed to use the Displacer Cannon to transport himself to the original Hazard Course, where Gina's hologram counterpart is still running in the part where ducking to go through pipes is taught. Interestingly, the damaged course is full of Vortigaunts, who attack the hologram, probably thinking it is a living being, but do not damage it. Shephard has to use the teleport located in the nearby observation room after coming across a dead security guard with a shotgun next to him and performing the ducking task. The observation room also includes an Easter egg image seen several times in Blue Shift.
In Decay, Gina's hologram counterpart is again seen when Gina Cross and Colette Green are required to cut through the destroyed remains of the Hazard Course to reach the surface with Rosenberg. Zombies instead of scientists are now found in the observation rooms.
Holography is also used by Black Mesa scientists for sending messages, such as a message addressed to Walter Bennet seen by Adrian Shephard in Sector E. Holography is also used to display other things, such as the holographic Earth used to display the satellite array status at the High Altitude Launch Center, as seen by Gordon Freeman.
Behind the scenes
For some reason, the Holographic Assistant model seen in Half-Life (used as the Holographic Assistant in the Hazard Course) and Blue Shift (used as Gina herself delivering the Xen crystal sample into the elevator, minus transparency) actually appears as an older woman with brown eyes, and without the bun pins worn by Gina. This model reuses the face texture of the Half-Life: Day One "player.mdl" model, sharing Gordon Freeman's and the Holographic Assistant model, with changes to make her appear older (and clearly named "olderface.bmp", while the young face is simply named "face.bmp"). The Decay Holographic Assistant model was however remade as a retcon to fit Gina's likeness, although the bun pins in the hair are still missing.