Philosopher's Stone

The Sorting Hat places Justin Finch-Fletchley into Hufflepuff House.

Chamber of Secrets

Introduced to Harry in Herbology on the first day of classes, Justin Finch-Fletchley makes no secret of his Muggle parentage in discussions with Harry. He is threatened by a snake during the Duelling Club, and as Harry was talking to the snake at that point, trying to calm it so it wouldn't strike Justin, it appeared to Justin as though Harry was actually egging the snake on. The Hufflepuffs become convinced that Harry is planning mischief against Justin in particular; when Harry goes to find Justin to explain what had happened, he finds that Justin has been Petrified by the monster in the Chamber.

When Justin is revived, he makes a point of coming over to Harry at the feast, and apologizing profusely for ever having suspected Harry.

Order of the Phoenix

Justin Finch-Fletchley is one of the original members of Dumbledore's Army, and is present at the first meeting in the Hog's Head. We see him occasionally in passing in DA meetings.

Justin is also one of the six members of Dumbledore's Army who see Draco Malfoy attempting to jinx Harry on the Hogwarts Express, and promptly return fire, leaving Draco, Crabbe, and Goyle looking like gigantic slugs squeezed into Hogwarts robes.