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 the Sim starts working out. If this is done, the coach may try again, or may target another Sim on the lot.
While coaches can be very irritating, a locked door can keep them out. However, a single-gender door will not stop a coach of the opposite gender. If the player can have the targeted Sims ignore or avoid the coach long enough, the coach will give up and leave. A mod, available here, will cause SentryBots to shock coaches.
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.