This locket belongs to Sherry Birkin and always in her possession. Its depict differently in each game.
Resident Evil (1998)
The locket is a circular, golden and have a big size pendant. Inside is a photo of her parents and herself. This locket belongs to her mother, Annette. When Annette realized an outbreak was going to occur, she implored her daughter to retrieve it from her jewelry box. Inside, there is a secret compartment that holds a finished sample of the G-Virus.
In the Claire B scenario of Resident Evil 2, Claire discovers T-00 is after the locket. She throws the locket into the molten metal, which the tyrant jumps after. This scenario is considered non-canon.
In the Leon B scenario of Resident Evil 2, Ada finds the pendant when Sherry accidentally drops it. She wears the pendant for the rest of the game, which causes T-00 to chase Ada and Leon through the police station, sewers, and Umbrella lab. After a discussion with Annette Birkin, Ada finds the G-Virus sample in a hidden compartment. She later escapes Raccoon City with the sample.
Resident Evil: The Darkside Chronicles
In Resident Evil: Darkside Chronicles, Sherry always wears the locket and never loses it; however, it is never mentioned if there was a G-Virus sample inside, and T-00 ignores Sherry presence attacking Leon and Claire. Moreover, The locket's design is a lot smaller than it was in Resident Evil 2. Although in an audio file, Annette calls Sherry at her school, and asks her to leave her school and take refuge at the Raccoon City Police Station because of the t-Virus outbreak. But before that, Sherry is asked by her mother to go at their home and take a "necklace" in the "white jewelry box on the dresser in mommy's bedroom."
Resident Evil (2019)
In the 2019 remake of Resident Evil 2, Sherry's locket no longer contains a family photo, simply having the name "Sherry" engraved on it. It also lacks a hidden G-virus sample; instead, it acts as a key to unlock the chamber in the P-4 Level Testing Lab containing the DEVIL Vaccine as well as some G-virus samples. The emblem on the vaccine's containment chamber matches the design on the back of the locket, acting as a clue to its true purpose.
Sherry is wearing her locket when Claire finds her. It is dropped when Sherry is pulled away by Brian Irons, leaving Claire to take it with her. Irons later calls Claire on a phone and demands her to bring him the locket in exchange for Sherry's life; however, he is killed by G before Claire arrives. It's unknown why Brian Irons seeks the locket; but it is implied that he may be aware of its use as the key to reach the DEVIL vaccine.
En route to the NEST, Sherry told Claire that the locket was given to her by Annette for her 11th birthday. Claire offered to give it back, but Sherry told her to keep it, admitting that she only ever wanted to spend more time with her mother on her birthday. Claire would later use the locket to access the DEVIL vaccine, which would ultimately be used to cure Sherry of her burgeoning G-Virus infection.