To what extent is Satan in Paradise Lost the prototype of the gothic hero?
The extent of the influence of Milton's Satan in Paradise Lost (1667) on the later Gothic novel (originated by Horace Walpole, The Castle of Otranto (1764)) was so great that nearly 100 years later (1850) the Romantic poets like Byron and Coleridge wrote about heroes who were called "Gothic Heroes" and came in three types. This is strongly indicative that the Romantics took their influence from Gothic novels which took their influence from Milton's Satan in Paradise Lost.
Ann Radcliffe, who popularized the Gothic novel after Walpole initiated them, developed the Satanic Hero modeled from the influence of Milton's Satan. She constructed Satanic Heroes who were often isolated; who aspired to attain secret knowledge or power (like the later Dr. Jekyll); who were fallen in some regard; who had a secret; who were dealing with personal psychological processes.
Ann Radcliffe is credited with being the inspiration behind the Romantic poets Promethean Hero and Byronic Hero (two of the three Gothic Heroes, Satanic Hero is the third), which are each subtle variations on the Satanic Hero developed by Ann Radcliffe and inspired by and sprung from Satan in Milton's Paradise Lost.