This passage is from an early section of Willa Cather's novel My Antonia, in which Jim Burden is first introduced to the character of Otto Fuchs. Jim is just a boy and is new to Nebraska, having just arrived by train from Virginia, and this new man impresses him to no end. Jim later learns that Otto is much nicer than he first appears.
Of course, all that information really doesn't answer your question! The previous poster's comments are good. I want to add that Jim's description of Otto shows Jim's boyishness. He compares Otto to a character from one his storybooks (Jessie James) and describes him using familiar stereotypes of the frontier (the desperado and the Indian).
The narrator conveys the way he feels about this character through the words he uses to describe him. He uses adjectives and descriptive phrases that portray the character as a scary individual.
For example, he describes the character as "ferocious" which is a pretty scary adjective. Similarly, he describes the way the scar curls his mouth as "sinister." He says the character looks like a "desperado" (a desperate outlaw) and an Indian. Finally, the top of his left ear is gone, which surely indicates that this is a man who is used to fighting and is, therefore, to be feared.