Han ska klä sig i modern stil, men det kan bara vara en illusion.
Från Biblioteket på Hurtfew (en utmärkt resurs för många saker Märklig och förstärkare ; Norrell):
His clothes are always spotless, fine and in the height of contemporary fashion, and he habitually wears a coat of bright leaf-green.
There is a suggestion however that the Gentleman's usual appearance may not be his natural one. At the end of the book, when he is fighting desperately for his life against Stephen Black, he begins to lose his humanity and appears more animal-like, with eyes further apart, snarling teeth and fur on his face. (It is clear from the footnote regarding the magician Simon Bloodworth and his fairy-servant Buckler that fairies were able to alter their looks at will.)
Jag försöker uppdatera detta svar med lämpliga bok citat om jag kan hitta dem.