Nemesis

From TF2 Wiki

Any player that kills you three times during a match without you killing them in turn or assisting in their death becomes your Nemesis, the event of which is accompanied by a distinct sound cue. The opposing player will see this as a Domination against you. When a player becomes your nemesis, they sport a distinct icon above their head which allows you to identify them (unless they are are currently a cloaked or disguised Spy). If your nemesis kills you, the Deathcam will identify them as your nemesis, with a short message that you were killed by [name] "again".

It is possible to have any number of opposing players as your nemesis at once. Upon killing a nemesis, you gain a Revenge kill, which counts towards your player stats, and the player loses their status as your nemesis.

The nemesis feature was added to promote relationships between players, creating a small negative reinforcement loop by giving players an opportunity to score extra points with a revenge kill.