Ramirez is a FEDRA patrol soldier who catches Joel, Tess and Ellie just outside the Boston wall. The ambush happens in the Outside section of The Outskirts, and it is the moment Ellie's infection scan becomes a problem.
Ramirez and her partner surprise the trio, force Tess and Ellie to kneel with their hands on their heads, and order a scan for infection. Tess tries to bribe the partner and is told to shut up.
When the scanner reaches Ellie, she stabs the partner in the leg. As Ramirez raises her rifle at Joel, Tess shoots her dead, and Joel kills the partner. The three escape into the trenches, and soon Humvees and troops are swarming the area searching for them.
The trio climbs a ladder out of the tunnels beneath an apartment block, and Ellie mentions it is her first time outside the wall. They spot a patrol in the distance but press on, and Ramirez's pair jumps Joel as he comes through a truck.
Ellie Williams
Ellie Williams is the 14-year-old girl Joel is hired to smuggle out of Boston, and your companion for most of The Last of Us Part I. She was bitten weeks earlier and never turned, so the Fireflies believe her immunity could lead to a vaccine. You play as her in the winter chapter and in the Left Behind DLC.
Joel Miller
Joel Miller is the man you play for almost all of The Last of Us Part I. A hardened smuggler in the Boston quarantine zone, he takes a job escorting 14-year-old Ellie out of the city, and the drop-off turns into a year-long trip across the country to find the Fireflies.
Tess Servopoulos
Tess Servopoulos is Joel's smuggling partner in the Boston quarantine zone and your companion through the first two chapters after the prologue. She is the one who talks Joel into taking Marlene's job, and she escorts Ellie with him across downtown Boston toward the Capitol Building.
Frank
Frank is Bill's former partner in Lincoln. You learn about him in Bill's Town through the Note from Frank, and he also turns up earlier in The Outskirts as the contact named in a smuggler's note.