The Coach is an NPC who appears in college sub-neighborhoods. A neighborhood will usually have three coaches, two male and one female. As with other University NPCs, it appears that the game assigns a specific coach to each college lot.
The coach will yell at a young adult Sim to stop whatever he or she is doing and work out, but that Sim may not do so right away, in which case the player can cancel the Work Out action before he or she starts working out. While coaches can be very irritating, a locked door can keep them out, though they can walk right into the other gender's bathroom/shower area with no hesitation. If the player can have his target ignore or avoid them long enough, they will give up and leave.
Supposedly, coaches target unfit Sims. In practice, they often target very fit ones, and have even been seen to target Sims who were performing fitness-building activities.
The Influence option will appear when clicking on a coach, but attempting to influence a coach seems to be ineffective.