Who is Rocky?

Rocky is an Eridian—an alien engineer from the 40 Eridani system—whom Ryland Grace revives aboard the Hail Mary. He and Grace learn to communicate (partly via science and a music-like language), become close collaborators and friends, and jointly develop a biological solution (Astrophage‑resistant taumoeba). Grace ultimately gives up his return so Rocky can take that solution back to save his homeworld.