A girl who, while she is a lovely person, is constantly cockblocking people, typically without realizing it. (Can simply be referred to as "Gwen." Male cockblockers are referred to as "Agravaine" or "Uther.")
Merlin Fan #1: I swear Merlin and Arthur were about to kiss in that last scene, but then Gwen walked in and ruined the moment.
Merlin Fan #2: Dammit Guinevere, you're such a cockblocker!