I guess you were right. Trying to leave this town will kill me. Still better than spending another day with you.
— Frank's note to Bill
Events of The Last of Us Edit
Bill described Frank as his former partner, "someone that he cared about". Despite this, Frank grew tired of Bill's methods of surviving much like Tommy was to Joel. Both of them had a "set-in-their-ways" attitude. Because of this, he left Bill, stealing many supplies and a car battery on the way to the truck at his hideout. On the way back, Frank was bitten several times by a pack of infected.
He hanged himself in his house so as to not succumb to the Cordyceps Brain Infection. Frank left a note for Bill telling him how he "hated his guts", growing tired of Bill's ways and how he managed the town.
After looting the area, Joel gave the note to Bill. (player-determined) Bill read it, disheartened at Frank's words sadly but kept up his bravado to get Joel and Ellie out of "his" town. Joel apologized for this after they got the car started and wished him farewell, since Bill was now truly alone.
Frank (Capitol Building) Edit
There was another Frank mentioned in the Smuggler's Note, which was found near a corpse in the flooded subway encountered just after the tragic experience at the Capitol building. From the context found in the Smuggler's Note, it seemed to be referring to the same person. This Frank was trying to enter the Boston quarantine zone illegally, and had procured false documents in order to do so, but ultimately never showed up to meet his contact to be taken to the quarantine zone.