Daniel Curien is the son of the infamous Dr. Roy Curien. In fact, he is the primary reason why Curien took part in Goldman's biological science project that soon became responsible for the creation of the undead plaguing the world.
Daniel makes his first appearance in the House of the Dead III (though he's mentioned in the Sega Saturn Manual of the first game), where his early life story is told in flashbacks in between chapters as shown:
First Flashback: Daniel is seen bedridden with what appears to be a terminal disease. His father Curien is sitting at his bedside saying that no matter what the doctors say, he will do everything to save his life, even if it means stepping into "forbidden territory".
Second Flashback: Curien is now leaning over Daniel, this time with hope in his voice as he holds a mysterious glowing canister given to him by Goldman (or perhaps directly by the late Clement Darling) which he refers to as "the key that will break the barrier between life and death" (which is either a compound of his own creation or that of Clement's). He assures Daniel that he no longer has to be afraid of anything anymore, not even death.
Third Flashback: Curien is sitting at Daniel's bedside again, contemplating what he's been doing lately, asking a quiet Daniel if he's gone over the limits of morality, since most of his researchers have left him.
Final Flashback: Curien rushes at Daniel's bedside, this time ridden by madness, possibly due to the nature of his experiments. In a maniacal manner, Curien claims that "he found it": genes that will supposedly change the future, referring to them as "The Magician" and "The Wheel of Fate", ending with diabolical laughter.
Daniel is finally seen in the present tense (though glimpses of him were seen in the intro movie and when he was walking towards a fallen Rogan) when he enters the room where Lisa, G, and Rogan are. After this meeting of fate, as the new "Player Two", he leads Lisa to the chamber where "The Wheel of Fate", which Daniel reveals to be a resurrected Curien, is being held. Though his speech while introducing the still-dormant creation to Lisa is confusing, it can be inferred that he disrupted the being's development, awakening it so he and Lisa can destroy it.
During this fight, the newly resurrected Curien seems to have no difficulty fighting his son, but when the being is mortally wounded after his defeat, he says to Daniel that he (Daniel) still needs him (Curien). At this, Daniel and Lisa ignore him and shoot the finishing shots with Daniel saying "You're not my father!". After the being explodes, Daniel says (in almost a sense of epiphany) that he has finally stepped out of the past, and has begun to walk towards the future.
Daniel's ultimate fate at the end of the game depends on the player's progress throughout the game. If one or both players do well, the normal and best endings depict Daniel showing no adverse effects of his life with his father and follows Lisa in leaving the area (with or without the jeep Lisa and G drove in with, depending on the ending). However, if one or both players did poorly, Daniel would stop moving and show little will to go on, prompting Lisa to witness him becoming a zombie and attack her, possibly due to the adverse effects of Curien's "cure" to his illness. It is unknown which of these is canon and whether or not Daniel is infected or, if so, whether or not the adverse effects are active (making him a zombie) or dormant (making him potentially become a zombie later).
Weapon: Franchi Spas 12 (It is "Uknown" Where Did He Get The Shotgun)
Status: Dead Or Alive '0_0
Dad: Dr. Roy Curien