Kaiser Wilhelm II is the closest to that person, even though him dying early probably just changes the dates of the war and the alliance setup. And with the alliance system, when it starts, everyone tends to get involved.
The problem is that is is 4 generations from the last Great European war, and people had forgotten the horrors of war. They also did not emotionally comprehend how lethal the newer weapons and industrial warfare would be. And the leaders like to play brinksmanship, and eventually it always blows up. Look at some of the chances for war.
1) Boer War - Germanic people see the crushing of free white settler colony/state with Horrific war crimes by the standards of the day.
2) A-H grabbing Bosnia 1908.
3) Italy carving off slices of the Ottomans 1912. It would only take one power getting involved to swing it into General war. For example, Russia moves on the straights.
4) Balkan Wars 1912-1913
5) First or Second Morocco Crisis. You can find lots of analysis on how a bad the Kaiser upset the UK, and people talk about how war could have been avoided, and this is possible. But is also possible if Germany finds a way of addressing the UK concerns, we could see WW1 early. All it would take is the Kaiser trying a partial mobilization to intimidate France, and 48 hours later, we could be at war. Or maybe the more neutral UK sees Morocco for the French Equitoral Africa as fair compensation and France begins to mobilization. A lot of the issues relate to the leaders not comprehending that mobilization was war.
Even if A-H does not reject the Serbian reply, and is instead talking internally on the reply, WW1 may still happen. Russian mobilization started 4 hours BEFORE A-H received the reply.
IMO, you have to change the leadership (not a leader) of 2-3 countries to make WW1 unlikely. You need a situation where war is not seen as desirable by the decision makers.