The Nemesis Project 

In any world, fantasy, sci-fi or reality-based, players need a strong character that they can put their wits and skills up against.  These other characters are nemesis'.  They are the bane of the players existence, they are the recurring nightmares that seem to be defeated but somehow survive to torture the player somewhere else some other time. 

The great thing about a nemesis is that it adds an air of competitiveness to the game. Not the unhealthy kind between PCs but between one character and his enemy. The nemesis is an ever-present threat to the character who now has the goal of defeating this enemy to aspire to. The first thing that needs to be thought about is what purpose a nemesis has in your game. 

Nemesis should be used to try and push your PCs a certain way; to put a new spin on the game. If your nemesis is working well you may influence the player to do something drastic to get back at him or her. For instance, if your nemesis was the mayor of a town, then not only would your PC hate that town but he may be tempted to contact the thieves guild in the town to try and frame the mayor. The PC might not normally consort with such lowly people but will because the player feels there is  reason behind it to do so.

A nemesis should be focused at aggravating the player, rather than  aggravate the PC. Some players don't get into their character enough to respond appropriately.  However, most of the time the PC’s behavior is largely that of the players. Thus, the the personality of the nemesis is of great importance for getting the player to become motivated.  

The nemesis' morals should be different than those of the PC, (mind you having a nemesis with the same philosophy  is often good and adds a twist – as with a nemesis coming from the same order of paladins as the PC) and most importantly, they should break many of the unspoken rules of engagement. Kill some of the PC’s family while he/she's in another country. Burn their hometown to the ground. Kill the age-old mentor. Do it all, and make sure when the PC finds out, its a slap in the face. If the nemesis has a signature mark, have him send the PC a letter containing nothing but the PC's sister's hairpin and his signature mark. Play cheap, whether that is evil or good. While an evil nemesis might murder a family, a good nemesis may be suitably powerful or honored to simply have the family imprisoned for some crime until the PC is 'brought to justice.'

The biggest challenge you face as an GM is to justify the actions of the nemesis.  Killing the family or loved ones of the PC (player) is often a great way to get the PC (player) to hate the Nemesis (which can often lead to a dislike of what you as an GM is doing to the PC.)

There are a number of events that you can use to justify the existence of the nemesis, besides just plain aggravating the PC and the player.

Inadvertent Events. The PC unwittingly interferes in the nemesis' business stopping him  from getting/achieving something very important to them. Now it’s payback.

Blast from the Past.  These are best used when a character has given you a background to the character.  However, this is not always the case.  Even without a background you can create an old foe (a defeated bully, a left-for-dead enemy) that has grown in power and now it is time for revenge.

On the wrong side of the law .  What is a law and what is not varies from group to group, culture to culture and in the world of many organizations, many which operate with only a cursory gesture towards the law, stoic adventurers who don't back down can find themselves making powerful enemies, and it is even more engaging when the organization has the support of the local region.

The Traitor.  There are few things more hated than traitors, especially when their true allegiances are shown in a fittingly destructive way for the PC.

Finally, a nemesis should not be just another adventure. A nemesis is never defeated until he's dead, because the hate should drive him to come back to fight once more and perhaps even back from the dead.