Both these characters are disaffected male teenagers who tell their own story directly (first-person narrative). They are at a particularly sensitive time of life, adolescence, when they find it hard to relate to other people: peers, authority figures, persons of the opposite sex.
On the face of it, Junior would appear to be worse off than Holden, being of a marginalized ethnic background. He is a poor Native American living on a reservation and sees many social problems affecting his community, such as the ravages of drink. He also has some physical problems from birth. Holden, from an affluent white background, does not experience such issues, yet he appears equally confused and worried by life.
The main similarity between the two characters really stems from the fact that they are adolescents trying to figure out their place in the modern world. However, they also resemble each other in some more specific instances. For example, they both experience the loss of a close family member. Holden loses his younger brother to leukemia (before the start of the story) while Junior’s sister Mary dies in an accidental fire. Junior also loses his grandmother to a drunk driver.
Holden and Junior change schools more than once - although in Junior’s case, he deliberately makes the decision to transfer to a white school, Reardan, whereas Holden seems to make a habit of getting kicked out of school as he doesn't apply himself to his studies. At school, they both also have run-ins with bigger, stronger boys: Roger in Junior’s case and Stradlater in Holden’s. A more light-hearted example is that they both appear to find history lessons very boring. Holden feels the need to apologize to his history teacher for not having paid attention in his class throughout the year, while Junior dozes off while his history teacher is droning on about the Civil War.