The Big Boss was an NPC originally intended to be included in The Sims 2. Not much is known about him, except he was going to have a role in the Boss scenario. Both him and the scenario were removed in the final release of the game, though his files are still there, albeit incomplete.
The strings viewed from SimPE show that the Big Boss scenario would have been very much like the Headmaster scenario. Sims would've invited him for dinner and given him a tour of their house, but would not have needed to schmooze him. Earning enough points to win the scenario would have granted a Sim a bonus of §750, while losing the scenario would have given Sims a job performance penalty. The big boss and his scenario were probably scrapped in favor of high-level career chance cards, many of which can grant a large amount of money, as well as a special memory.
It should be noted that he can be seen when calling in sick, but this generates a new NPC instead of the Sim shown in the thumbnail. Also, his gender is ambiguous; if summoned, the Boss NPC has the opposite voice of what he/she appears to be (e.g. a male has a female voice and is treated as female). Looking in SimPE shows that he would have arrived in a helicopter.