My trilogy features two important characters that I wish to keep the identities of hidden. The first is the series' deuteragonist, who is heavily implied to be Jeanne d'Arc (yes, THAT Jeanne d'Arc), who for reasons unknown decided to use her sister's forename and mother's maiden name as a pseudonym to disguise her identity from the main protagonist. Jeanne's identity eventually revealed via a series of flashbacks/dream sequences written from the protagonist's perspective that recount major events from Jeanne's life, which are completely devoid of dialogue (because I have no real way to justify why characters featured these flashbacks would be able to speak perfect modern English). Although, I'm trying to not make the protagonist look like a complete and utter fool because one of his major traits is his in-depth knowledge of world history and mythology and a major plot point is him trying to discover Jeanne's identity.
The second character is Loki, who acts as the trilogy's secondary antagonist and throughout the series, the audience is given hints about his identity. For example, Loki goes by the name of Hveðrungr, until he reveals himself towards the end of the series and several of the Norse myths are sometimes alluded to in several conversations.
How, then, can I disguise the identities of these characters?
EDIT: This trilogy of mine is set in modern time