The phrase "mysterious young man" is an idiomatic expression that refers to a person who is enigmatic, elusive or difficult to understand. Full definition
It should be noted that Jordan is playing, not the protagonist but an antagonist, an initially mysterious young man who is first seen in association with brazen criminal Orlando Klaue (Andy Serkis). (dallasfilmnow.com)
Most new testament scholars say that the Gospel of Mark originally ended with the story of the women who go to the cemetery, only to encounter a mysterious young man pointing to Jesus» empty tomb and announcing the resurrection. (religion-online.org)
Some scholars have called this mysterious young man a symbolic representative of the just - scattered disciples (who also «fled,» as would the three women from the tomb). (religion-online.org)