Ok well, my reasoning for leaning towards ISFJ for Neville is the loyalty he displays towards his friends and general reluctance he has towards sticking up for himself. While this could also be an INTP trait, it seems more predominately the behavior of a young and overly sheltered ISFJ to me. He also shows little aptitude for potions which seems to be a highly theoretical subject. Hm, no well I suppose sent it would be taught in a classroom I could see it being more regimented and exacting.
And I suppose that his inferior Fe could push up out of his control and cause him to be ridiculously nervous which could cause a potential disconnect between his brain and his body. Yeah I suppose an ISFJ would probably have that more under control, and they would probably be better able to follow the precise and detail oriented curricula that teaching the bare basics the practical aspect of potions.
And the Neville in the books always seems to give off that detatched and yet slightly frantic air that I would theorize would be more prevalent amonst Ti-doms, especially when paired with Ne. And his near mastery of Herbology, something of interest to him, also is a point towards INTP... Ok you convinced me I change my mind, I would type Neville as an INTP.