The phrase "mysterious new character" refers to a person who is not well-known or familiar, and has an air of mystery surrounding them. Full definition
Deep Strange Journey will feature the introduction of a mysterious new character named Alex, a new ending path, new artwork, and more, according to a translation by Gematsu. (gamespot.com)
The show is reportedly looking to cast a mysterious new character called Moreau in the season finale. (io9.gizmodo.com)
Laura Dern's mysterious new character, Resistance Vice Admiral Amilyn Holdo, is introduced, while there are also contributions from familiar cast members such as John Boyaga and Daisy Ridley. (highsnobiety.com)