Victor Frankenstein describes his younger brother:
Ernest was six years younger than myself, and was my principal pupil. He had been afflicted with ill health from his infancy, through which Elizabeth and I had been his constant nurses: his disposition was gentle, but he was incapable of any severe application. (1.1.10).His health improves, however, during Victor's long absence at Ingolstadt (1.5.1). In the first edition Elizabeth and Alphonse propose that he should become a farmer -- "the least hurtful, or rather the most beneficial profession of any" (1.5.2). By the time of the third edition Ernest has become more extroverted. Elizabeth reports: "He is now sixteen, and full of activity and spirit. He is desirous to be a true Swiss, and to enter into foreign service" (1.5.1).
Ernest is the only Frankenstein to survive the novel (3.6.5).