Neville's Big Baby Care Diary is a book found by Luigi in the Study of the mansion. It was written by Neville himself. A passage from it describes how his third son, Chauncey, has been scaring people. However, he says that the twins, Henry and Orville, were never like that; they are afraid of ice (cold), fire, water, and even wind. This acts as a guide later on when Luigi defeats Chauncey and the twins. He concludes by telling the reader that the twins act and (metaphorically) are more like younglings than Chauncey is.
"My third son is still a baby. Oddly enough, he seems to scare people in ways the other two never did... The twins, now... They're afraid of fire, water, cold, and wind! Why, they shake with fright when beset by the elements! Sometimes I think they're more childlike than the baby!"