Isabella's Lullaby

Isabella's Lullaby is a song Leslie wrote and played on his ukulele. He was originally embarrassed by it and did not play it for anyone, except for Isabella. Isabella fell in love with the song, and continued to hum it years after Leslie's death.

Plot
Isabella hummed the song as she was walking Conny to her death. Conny asked Isabella what song it was, but did not receive an answer, implying that Isabella did not normally hum it around the children.

It is later revealed that Isabella hummed the song while she was pregnant with Ray. Because Ray did not experience infantile amnesia, he remembered the song and began to hum it to himself. This eventually led Isabella to discovering that Ray is her biological son, as none of the other children had heard the song before.

Trivia

 * While referred to as "Isabella's Lullaby" in the official soundtrack, the in-universe song has no official name.
 * A video of Isabella's Lullaby is currently the most viewed Promised Neverland-related video on YouTube, with over 27 million views as of July 2022.
 * The Season 1 original soundtrack cover shows Leslie playing the song for Isabella.