In a medieval European context, Jehan is indeed one of the many forms of
John or
Joan.
However, it has a separate origin as a Muslim name, going back to a Persian world meaning "world", though it is also used in Arabic speaking countries like Egypt. Jehan Sadat is the widow of the former President of Egypt,
Anwar Sadat.
http://www.behindthename.com/bb/arcview.php?id=281787&board=gen
Not having read
Inkspell,, I don't know whether a medieval European or a Muslim origin of the name is more appropriate in that case.