The unnamed captain is a police captain who appeared in "Welcome to Storybrooke".
After the Curse
Owen is found by the captain. ("Welcome to Storybrooke")
In 1983, after the Dark Curse has brought people of various lands to Maine, some time after leaving Storybrooke, young Owen Flynn (who would grow up to become Greg Mendell) returns to the site where he'd run off from the evil Mayor - leaving his father behind -, accompanied by two police officers. However, he can't find the town. Owen turns to them and says that this is where they took his dad, but the police captain tells him that there is nothing there. Owen insists that this is where Storybrooke was, by the rock next to them. The captain tells him that there isn't a town called Storybrooke anywhere in Maine. Owen asks if he doesn't believe him, and the captain squats down before him and simply comments that he's been through a lot. Owen is further frustrated and runs away from him. He stops shortly afterwards and starts shouting, "I'll find you, dad! I promise! I'll never stop looking!", all of this as he holds up the lanyard given to him by Kurt. We can then see that he is just outside of the Storybrooke town line, and Regina is actually standing right in front of him, looking depressed. She reaches out her hand, wanting to touch him, but it rubs the magical barrier protecting the town from outsiders. The police officers approach the sobbing Owen from behind, and the captain tells him to come with them. Owen looks back once more as he is taken back to the police car, and a tear slides down Regina's face. ("Welcome to Storybrooke")