Telomere is a vampire organization that's goal is to corrupt human society and kill Mina Tepes. It is lead by Katie Maurice. Both the anime and manga show that humans are also affiliated with Telomere. Mr. Isurugi is shown to be allied with them for the Pied Piper to allow public opinion of vampires to sour and force them to hide again.
The exact size of the organization is unknown. But it appears Katie has nearly enough resources as Mina.