The Mystery Man is a fanon term to describe the unknown man in a business suit who walks with a noticeable limp. He has only appeared an ending for The House of the Dead III, and again for the fourth game.
He has appeared in one of the endings of The House of the Dead 3 where he walks in the chamber where The Wheel of Fate was recently defeated and picks up a glowing canister that was seen earlier in a flashback in the hands of Curien. As he holds it in his hands, he chuckles to himself saying, "It appears that he didn't understand its true purpose," and then walks off.
He also appears in one of the endings in the fourth game where he's in an office, saying that Goldman was soft and humans had no need for hope. He also mentions that there's more than one "Pandora's Box" (a chamber where undead creations are held prior to release).
Not much is known about the Mystery Man; (even this name is unofficial) and as such his true identity is still unknown. There have been some brief glimpses of his face however, during the ending in "The House of the Dead 3" as he picks up the canister, and during the ending in "The House of the Dead 4". He may be the successor Goldman was talking about at the ending of "The House of the Dead 2". There have been a handful of guesses of who he is, but one of the most prominent and likely guesses is Harry Harris from The House of the Dead 2, due to his curious behavior and his injury by the boss Strength.
Some gamers think that the Mystery Man is Goldman. However he mentioned in one of the endings in House of the Dead 4, that Goldman was "soft", meaning he and Goldman were separate beings.
There is also a possibility that the Mystery Man is Papa Caeser from "The House Of The Dead Overkill". Although this points more towards the unlikely side, as apparently Ceaser was killed by Clement Darling on an electric chair at the climax of the game. However, one certain fan has posted a video on YouTube with some interesting footage (see here: http://www.youtube.com/watch?annotation_id=annotation_861682&feature=iv&v=gJPYXKdlFHw) and his input (among other theories) providing some possible support to this theory. Among the footage presented is a close-up look at the Mystery Man's face in one of the endings of "The House of the Dead 4", which has a considerable resemblance to Papa Caeser's face. The video also provides a clip from "Overkill" showing Caeser's leg being shot as he's getting off a vehicle. If this theory is true, it could mean that Caeser somehow survived the shock from the electric chair, or was somehow cloned using the mystery compound and/or the sciences responsible for causing the mutant mess in "Overkill". This video also proposes the possibility that the Mystery Man could be Issac Washington's father, who is revealed to be alive by Papa Caesar's vocal recording heard at the end of "Overkill".
The Mystery Man will probably appear in a future installment, revealing his true identity at long last.
- He appears different and slightly older in House of the Dead 4 ending compared to what he looked like from The House of the Dead III ending.