Character Analysis
Messner's an awfully competent dude, but he's been dealt a bad hand in this situation. Like, not as bad as the people living as hostages—but only until the terrorists and hostages become buddies in a beautiful utopian community world.
Back to Messner. A Swiss guy who's done well in his job with the Red Cross, he's pulled into all the action because he happens to be vacationing nearby when everything goes downhill. All he wanted to do was take a nice vacation in South America and peek at a few archaeological ruins, and now he's stuck negotiating a hostage scenario. He turns out to be the negotiator the terrorists like working with, so he ends up staying through the months of the hostage crisis. Lucky guy.
Objective(ish) Observer
Messner is one of the few characters who's removed enough from the situation to be reasonably objective about it. Sure, he's involved, and he has some hopes and perspectives, but he's a lot less in the thick of things than the hostages or terrorists or even the country's government. He's working to make sure the hostages are okay, but other than that, he tries to keep neutral.
The book is pretty up-front about this. The narrator explains, "The sign of the Red Cross, like the very sign of Switzerland, stood for peaceful neutrality. Messner had stopped wearing his armband a long time ago but he didn't believe in it any less" (10.34). The same section explains that Messner isn't a spy, and he's not going to tell either the government or the terrorists what the other side is doing without permission. He's just going to try to negotiate the best outcome for everyone.
Because Messner has a more objective role than most of the characters, and because he's the one character who has regular contact with the outside world, the narrator often conveys information by telling us Messner's thoughts or perspectives. For instance, it's from Messner's thoughts that we learn the military is actually building a tunnel to invade the house (8.30).
Messner is also the character who injects a sense of urgency into the last chapter, which is what lets us readers know that something big is about to happen. As everyone else gets lost in enjoying the world they've come to know inside the house, he keeps insisting that something needs to be negotiated fast. Nothing like a sense of foreboding to put those happy community scenes in a new light.
Messner also gives some added perspective on the terrorists. As someone who knows a lot about tricky international situations, he confirms that they're different from other terrorists he has encountered:
More than any other negotiation Messner had ever been involved with, he found that he didn't really care who won this one. But that wasn't it exactly, because the governments always won. It was that he wouldn't mind seeing these people get away, the whole lot of them. (8.30)
Pretty decent guy, right? Plus, Messner's recognition that these terrorists are different from others he's encountered makes the whole world developing inside the house a little more believable. So basically Messner helps convey a lot of information, he's a pretty likable guy, and he helps keep the plot from turning into a utopian fantasy through the occasional reality check. He's kind of like the holes in the Swiss cheese.