(covers information from several alternate timelines)
"That's one way of putting it."
The temporal agent known only as Daniels was an operative from the 31st century and a major player in the Temporal Cold War. Daniels had frequent interactions with the crew of Enterprise NX-01 in the 2150s and often assisted them in preventing temporal incursions by various factions in the Cold War.
Encounters with Enterprise
Assignment to Enterprise & Apparent Death
Daniels first came into contact with the Enterprise crew in 2151, when he infiltrated the ship posing as a crewman, and shared quarters E-14 with an Enterprise crewman. Working undercover as a steward, Daniels had been assigned to Enterprise to capture Silik, the leader of the Cabal, who they knew would be boarding the ship at some point. Silik did indeed board Enterprise several months into their mission, posing as a Borothan pilgrim, at which point Daniels revealed himself and the true nature of his mission to Captain Archer. While Daniels and Archer worked together to capture him, Silik confronted Daniels and shot him with a particle weapon, apparently killing him. (ENT: "Cold Front")
Destruction of Paraagan II
Despite this, Daniels reappeared the next year, taking Archer back in time ten months, in order to have a conversation with him in secret from the other factions in the Cold War. When Archer expressed surprise at Daniels' survival, believing that Silik had killed him, Daniels only responded, "He did... in a manner of speaking." Daniels instructed Archer in the construction of two extremely advanced quantum beacons, which the Enterprise crew used to detect a cloaked Suliban stealth cruiser, and prove that the Cabal had attempted to frame Enterprise for the destruction of the Paraagan II mining colony. Afterwards, when Silik threatened to destroy Enterprise unless Archer handed himself over, Daniels transported Archer forward in time to the 31st century, in an attempt to protect him. Daniels' actions had disastrous consequences, however, in that removing Archer from the timeline changed history in such a way as to destroy Daniels' civilization. With no way to return Archer to his own time and restore the timeline, Daniels and Archer were forced to improvise, using Archer's communicator and hand scanner to construct a makeshift temporal communicator, which Archer used to communicate with his crew in the 22nd century. Archer's crew managed to retrieve a piece of technology that Daniels had left in his quarters on Enterprise, and trick Silik into interfacing it with the technology that he used to communicate with the Cabal's benefactor in the 28th century. This temporarily turned Silik's technology into a means by which Archer could return to his own time, defeating Silik and correcting the damage to the timeline. (ENT: "Shockwave", "Shockwave, Part II")
Xindi-Reptilians on 21st Century Earth
In 2153, Daniels returned to Enterprise, which was in the midst of searching for the Xindi weapon in the Delphic Expanse, to inform Archer that they had detected three Xindi-Reptilians on Earth in the year 2004. As Daniels could not secure authorization from his superiors to travel back in time himself, he offered to send Archer back in time to stop the Xindi, who were attempting to synthesize a bio-weapon in secret, with which to destroy Humanity. He supplied Archer and T'Pol with temporal tags to mark anything that did not belong in that time period for retrieval, and a temporal beacon with which to signal Daniels when their mission was complete. After the successful completion of their mission, Daniels returned Archer and T'Pol, along with the Xindi bodies and all of their equipment, to Enterprise, only moments after they had originally been transported back in time. (ENT: "Carpenter Street")
Attempting to stop Archer's Suicide Mission
The next year, Daniels returned to plead with Captain Archer to not undertake a suicide mission to destroy the Xindi weapon, explaining that Archer's role in history was too important for him to sacrifice himself. To make his point, Daniels transported Archer to the USS Enterprise-J, a Starfleet vessel in service during the 26th century, so Archer could witness the historic Battle of Procyon V, wherein the forces of the Federation defeated the Sphere-Builders. Daniels explained that the Sphere-Builders were attempting to manipulate the Xindi into destroying Earth, so that the Federation would never come into existence, and they could invade the galaxy unopposed. Daniels attempted to convince Archer that the only way to defeat the Sphere-Builders was to form an alliance with the Xindi, but Archer refused, unwilling to risk the destruction of Earth. In the hopes that Archer would change his mind, however, Daniels supplied him with a Xindi initiation medal which belonged to one of the crewmembers on the Enterprise-J, revealing that, in the 26th century, the Xindi were members of the Federation. The medal later proved to be an important component in saving Earth, as Archer used it to convince the Xindi-Primate Degra that Humans and Xindi were destined to form an alliance. (ENT: "Azati Prime")
Alternate World War II
Later that year, as Captain Archer was preparing to lead a team to board the Xindi weapon and destroy it, Daniels transported him to 2161 to witness the Federation Charter being signed. Attempting to convince Archer to send someone else on the mission, Daniels explained that, without Archer, the Federation would never come into being, and that it was too dangerous to risk his life. Archer refused, however, feeling that it was his responsibility to destroy the weapon. Later, with Archer trapped on board the exploding weapon, Daniels transported him and Enterprise back in time to 1944, in an alternate timeline created by multiple temporal incursions by various factions in the Cold War.
In this timeline, the Nazis had launched a successful invasion of the American East Coast and had formed an alliance with the Na'kuhl, a Cold War faction from the 29th century. Daniels appeared in Enterprise's sickbay, dying from a condition which caused various parts of his body to exist in different temporal states. He explained that the Temporal Cold War had ignited into a full-blown conflict and that different temporal agents stationed throughout the timeline had been given orders to change history.
This resulted in temporal anomalies ripping through the timestream, causing Daniels' condition. Before dying as a result of his injuries, he informed the crew that the conflict was initiated when the Na'kuhl launched an attack on the 31st century using a temporal conduit they were constructing with the assistance of the Nazis, and that he had transported Archer and Enterprise to this point in time to stop them. Eventually, Enterprise was successful in destroying the conduit before the Na'kuhl had a chance to use it, an action which had the effect of resetting the timeline and reversing Daniels' death.
Afterward, Daniels returned Archer and Enterprise to their proper place in time, but not before Archer informed him that he was done being a pawn in the Temporal Cold War. Bidding Archer farewell, Daniels told him that, as a result of his actions, the Cold War was coming to an end. (ENT: "Zero Hour", "Storm Front", "Storm Front, Part II")
In the script of "Cold Front", Daniels was initially described thus; "He's early 30's, curious but deferential."
In the final draft script of "Shockwave", a note regarding Daniels stated, "He appears as he did in 'Cold Front'." The black costume that was worn by Daniels made its debut in "Shockwave" and was referred to, in the script, as "a futuristic uniform".
Daniels appeared in an early version of the script for "Future Tense". When Mike Sussman pitched the story to Brannon Braga, Sussman planned for Daniels to materialize on the bridge of Enterprise at the end of the narrative, planning to retrieve an Earth vessel. The pod featured into the final version of the installment, containing an unidentified deceased body. In response to Archer asking whose corpse was inside the vehicle, Daniels would have revealed that the dead body was his own. He then would have disappeared, leaving Archer with no more answers. Sussman wasn't sure why this story point was ultimately excluded from the plot, but reckoned that the decision was made "at a high level" to keep the episode more mysterious. As a result, Daniels was written out of the tale, and at the point when he would have appeared, the pod instead simply disappears, which Sussman believed was just as effective. He recalled that his initial notions about the identity of the dead body was either Daniels or a similar time traveler. 
In the final draft script of "E²", Daniels was referred to when Captain Archer, having found that Enterprise had accidentally traveled back in time, rhetorically asked T'Pol, with grim humor, "Where's Crewman Daniels when you need him."
In the final draft script of "Storm Front", Daniels was initially described thus; "His mangled face is a horrifying sight... as if patches of ancient, decaying flesh have been grafted onto his skin. Clumps of his hair are missing from his scalp... his right eye is pale, almost solid white."
In a potential story pitched by Mike Sussman, Daniels would have come aboard Enterprise and brought Chef, played by William Shatner, into the future, making sure history took the correct course by having him pose as Shatner's original character during a significant event.
While Crewman Daniels appeared to be allied with the United Federation of Planets, Daniels has explicitly stated, in "Cold Front", that he is not a Starfleet officer.
Daniels also appears in Star Trek Online, voiced by Matt Winston. In the Agents of Yesterday expansion, he masquerades as "Ensign Hunter", assigned to the crew of a 23rd century starship commanded by the player character, in order to prevent the Temporal Liberation Front – an alliance of Na'kuhl, Vorgons, Krenim, the Sphere-Builders, and later the Terran Empire, all opposed to the Temporal Accord – from altering history in that time period. After the player is "killed" at the Battle of Caleb IV, Daniels recruits the player as a temporal agent, and brings them forward to the year 2409 (the year the Star Trek Online story begins) to combat the Na'kuhl in the game's "present". Daniels begins suffering progressive scarring from temporal flux linked to "the Envoy", the leader of the TLF. He is killed by the Envoy - revealed to be the Krenim scientist Noye - at New Khitomer. After Noye and his forces are defeated at Procyon V, Daniels' death is reversed, just as it had been at the end of "Storm Front, Part II", and bids farewell to the player. Daniels later joins with Sela and the player character to discover the true fate of Natasha Yar after she went back with the USS Enterprise-C. Daniels returns in the Age of Discovery expansion to greet temporally-displaced Starfleet characters "killed in action" during the fall of Starbase 1.