The character of the Ancient Mariner is presented to us on two levels. There is the external impression that we get of him from the Wedding Guest's viewpoint as a physically imposing, rather intense and terrifying figure; the Wedding Guest is rather scared of him. But we also get his own story, from his younger days, in which he appears initially as a thoughtless and rather hot-headed man, shooting an albatross during a voyage at sea, for no real reason. The terrifying supernatural events that follow this, however, have the effect of transforming him and making him realise that he should think before he acts and treat all living things with respect. Overall, the impression we have of him is that he is a wise (if rather intimidating) elderly and experienced seafarer who has learned an important lesson, which he is eager to pass on to members of the younger generation, like the Wedding Guest.
The Anciient Mariner is an "Everyman" figure who in one impulsive act transforms the course of his life. With the death of the Albatross, the Ancient Mariner faces internal conflict over the crime he has committed, and embarks on a journey to understand his actions and perform his penance. His journey is one of self awreness and provides lessons in personal accountabilaty, acceptance, and forgiveness. The Mariner offers no explanations or excuses for the killing of the Albatross simply because he has none and has come to accept that. The Mariner ultimately learns his lesson, but is forced to eternally seek penance in the teaching of the lesson.