Necklace | NANA | Ren's Lock & Nana's Key

Like Sid Vicious, Ren wears a chain with a lock around his neck that has the initial, "R" on it, to which only Nana has the key.

Nana (ナナ), often stylized as NANA, is a Japanese manga written and illustrated by Ai Yazawa. The manga was published in Shueisha's manga magazine Cookie. Viz Media licensed the series for release in North America and published it in their Shojo Beat magazine. In 2005, Nana was adapted into a live-action film starring Mika Nakashima and Aoi Miyazaki. Nakashima returned to star in the 2006 sequel, while Yui Ichikawa replaced Miyazaki. That same year, an anime based on the manga was produced by Madhouse Studios.

The story follows Nana Komatsu and Nana Osaki, who meet coincidentally on a train to Tokyo. They become roommates and develop a close friendship.

(from Fandom NANA Wiki)