Who first accused Peter of having been with Jesus?

Answer: A servant girl

As Peter sat in the courtyard, a servant girl said, "You also were with Jesus the Galilean," but he denied it before them all.

“Meanwhile, Peter was sitting out in the courtyard, and a servant girl came up to him. “You also were with Jesus the Galilean,” she said. But he denied it before them all: “I do not know what you are talking about.””— Matthew 26:69-70 (BSB)