Captain Sidekick can be unlocked during Chapter XIV of A Plague Tale: Innocence. You need to be faster than captain during the section where you have to follow him. Read this chapter to learn how to do that.
You have only one chance to get this achievement/trophy. At some point, the captain will go out into the yard. He stops there to talk. When that happens:
- get out from under the naves and turn left
- go along the hedge and turn inside the area that is surrounded by it
- go through the second arch, which is adjacent to the wall
- turn left and enter the corridor
After performing these actions, you will unlock this trophy/achievement.