Riley's Firefly pendant starts Lakeside Resort already in Ellie's backpack, but it only counts once you open the backpack and look at it. It belonged to Riley Abel, Ellie's closest friend from Boston.
Ellie keeps Riley's pendant with her, and when you inspect it she says she misses Riley. Left Behind shows how Ellie came to have it.
Braun's Firefly Pendant
Braun's Firefly pendant rests on a bench inside a gazebo next to the first cabin at the resort, in The Hunt section of Lakeside Resort. It belonged to Paul D. Braun.
Ellie's backpack
Ellie's backpack is an automatic artifact Joel picks up in the back of the butcher shop in Lakeside Resort. As he walks in, he grabs it from a low shelf, and it stays in his Artifacts menu from then on.
Joel & Sarah photo
The Joel & Sarah photo is a picture of Joel with his daughter, an automatic artifact in Ellie's backpack at the start of Lakeside Resort. You can view it from the Artifacts menu while playing as Ellie.
Kristof's Firefly Pendant
Kristof's Firefly pendant is inside a blue storage container in the coal mine during The Hunt section of Lakeside Resort. It belonged to Travis Kristof.