Race engineer
The race engineer is the one person who speaks to the driver on the radio, translating the pit wall's decisions into what the driver needs to hear.
Every "box, box" and every gap update comes from them. Behind that, they own the car's setup across a weekend and act as the driver's single filter onto a large engineering group — which is why the relationship tends to follow a driver between teams.
Much of the skill is knowing what not to say. A driver mid-stint has limited attention, and the engineer decides what is worth spending it on.