The subtitle “The Story of a Buried Life” gives you one clue as to who the angel is. The angel of the title appears in the book in the form of a stone angel that Gant sells to Eliza to use for a gravestone of a poor woman. Gant became a stonecutter when he was not such a horrendous person, but his behavior spirals downward throughout his life and he is an abusive alcoholic. The angel in the story is Eugene, who is often described as angelic or ethereal. He is introspective and reflective beyond his years from the very beginning, but he never really realizes his potential. He may lead a buried life because he has trouble moving beyond his upbringing despite his heavenly potential. He constantly looks homeward until the final moments when he leaves Ben in the graveyard and turns to leave Altamont.
Actually, the title "Look Homeward,Angel" comes from the John Milton poem "Lycidas" with the line "Look homeward, Angel,/now, and melt with ruth,"...the novel was originally supposed to be titled "Oh, Lost!" but Maxell Perkins and others at Scribers veto-ed it. The title now refers to the journey of the narrator to reconcile the death and subsequent journey of the title character.
Ultimately, it's a message of hope.