The speaker is the Ancient Mariner who has stopped a man in order to gain an audience for his story. He was a man who travelled the seas and had a wealth of stories to tell.
The poem is filled with a tone of dread and despair, particularly after the mariner has shot the albatross. The horrible images he sees of his dead and dying fellow crewmembers, the terrible thirst and pain that assails him as well as the spirits from the underworld that visit him strengthen this tone throughout the poem.
The tone is also supported by the fact that the narrator was quickly left alone on the ship and was forced to deal with the loneliness and terror that assailed him.