(covers information from several alternate timelines)
"Don't be concerned with 'when'."
The Humanoid Figure was a mysterious individual who acted as benefactor to the Suliban Cabal in his role as a representative for a faction in the Temporal Cold War. Unable to physically travel through time, he could only partially materialize to deliver information from the 28th century. As payment for their services, he provided the Cabal with the technical expertise to perform genetic enhancements on themselves, including the ability to shapeshift, enhanced senses, and the ability to survive in extreme environments. He also gave them advanced technology such as biomimetic garments and cloaking devices.
Plots of his included staging attacks on the Tandarans, trying to start a civil war within the Klingon Empire, and sending Silik, leader of the Cabal, to steal a 31st century Temporal Observatory. (ENT: "Broken Bow", "Cold Front", "Detained")
In 2152, he ordered the Cabal to frame Enterprise for the destruction of the Paraagan II mining colony. When Captain Archer stole evidence of this plot from a Suliban stealth cruiser, he ordered Silik to bring Archer to him. In an attempt to protect Archer, the 31st century temporal agent Daniels transported Archer forward in time nine hundred years. Daniels' actions had disastrous consequences, however, in that removing Archer from the timeline changed history in such a way as to destroy his own civilization. This also had the effect of removing the Cabal's benefactor from the timeline, as Silik discovered when he unsuccessfully attempted to contact him. Eventually, Archer was able to return to his own time, restoring the flow of history. (ENT: "Shockwave", "Shockwave, Part II")
In 2153, after Earth suffered a devastating attack which resulted in the deaths of seven million people, he ordered Silik to retrieve Captain Archer and deliver him to a temporal chamber. There, he provided Archer with intelligence that the Xindi were behind the attack, and that they were working for another faction in the Temporal Cold War. This gave Starfleet the information it needed to mount a mission to stop the Xindi from destroying Earth. (ENT: "The Expanse")
One year later in 2154, he assigned Silik to stow away on Enterprise, when he learned that Daniels would be sending it back in time to 1944 to stop the Na'kuhl, another Cold War faction, from constructing a temporal conduit. Wanting the ability to physically travel through time rather than just communicate through it, he ordered Silik to steal the conduit's specifications and deliver it back to him. Silik was discovered, however, and eventually teamed up with Captain Archer in order to stop the Na'kuhl. They were eventually successful in destroying the conduit before the Na'kuhl had a chance to use it, but Silik was shot to death in the process. The defeat of the Na'kuhl and the destruction of the conduit led to the end of the Temporal Cold War. (ENT: "Storm Front", "Storm Front, Part II")
The Humanoid Figure, aka "Future Guy", was played by actor James Horan.
Star Trek: Enterprise creators and Executive Producers Rick Berman and Brannon Braga invented this character without deciding his identity. (Broken Bow, paperback ed., p. 258) "Because so much of the show's concept is rooted in Star Trek's history, we thought it would add a little spice to create a villain who doesn't want to see humans succeed in space," recalled Braga. "It was just too cool a premise to pass up." (Star Trek: Communicator issue 139, p. 25)
In the series bible for Star Trek: Enterprise, this character was referred to as "an unseen leader [of the Suliban] from the distant future" and "someone from a distant century who is providing them with technology in exchange for [...] [something which] involves a 'Temporal Cold War'." The ENT bible also likened the character to "the devil" (as the Suliban were referred to as having made a "deal with the devil"). Silik actor John Fleck once also made the same comparison regarding the Humanoid Figure (as Fleck believed Silik's devoted attitude towards the mysterious person was similar to Silik "selling his soul to the devil"). (Star Trek: Communicator issue 139, p. 41)
In the revised final draft script of ENT pilot episode "Broken Bow", the Humanoid Figure was initially described as someone whose image was barely discernible. A subsequent scene description in the same teleplay elaborated, "The Humanoid Figure [...] is a male of indeterminate age; we can barely make out his appearance or his words." 
James Horan had little idea about precisely who this character was, apart from that he was from the future and was using the Suliban to manipulate the past. In fact, Horan was given only pages of the character's dialogue, rather than the full script of "Broken Bow". At a time when the character had appeared in no other episodes but that series pilot, Horan recalled, "They weren't telling me anything, so I had to pull what little info I could [....] At the audition I said, 'Who is this guy?' and they just said, 'We don't know!' I said, 'Well, is he a hardass; is he a commander? What does he do?' And they said, 'Yeah!' So I gave it a go, and I said, 'Something like that?' They said, 'Yeah, exactly like that!' I gave him a bit of a stentorian tone to his voice because the lines seemed like he was in charge, and he was admonishing this Suliban to keep him in line. I just played him with a little bit of an edge, I suppose." (Star Trek: The Magazine Volume 2, Issue 10, pp. 44-45) For "Broken Bow", Horan spoke the character's dialogue on the set. (Star Trek Monthly issue 91, p. 32)
After Enterprise entered its first season, Rick Berman and Brannon Braga remained unsure about the Humanoid Figure's identity. At the time, Braga admitted, "We have several possibilities. But we have not settled on any of them and we may come up with yet another one. I think we're going to see how it plays out…. We have some ideas, but honestly we don't know for sure. We'll find out along with Archer." (Broken Bow, paperback ed., p. 258)
This character was known only as "Humanoid Figure" and continued to be referred to as that in the episode scripts, as well as in press releases. Given the character is from the future, fans began referring to him as "Future Guy". ("In Conversation: Rick Berman and Brannon Braga", ENT Season 1 Blu-ray special features) He is also sometimes called "Doctor Claw" in some fan circles, after an Inspector Gadget villain (voiced by Frank Welker) of the same name who speaks in a distorted voice and, despite appearing in every episode, is (almost) never clearly shown.(citation needed • edit)
John Fleck once remembered how the Humanoid Figure was filmed for the character's reappearance in "Cold Front"; "It was a stand-in, and he was just standing there while somebody read the lines offstage because he wasn't going to be reading the dialogue. You can't really see his lips move." Concerning both the actor who played the Humanoid Figure on the set for "Broken Bow" and the stand-in who substituted for "Cold Front", Fleck concluded, "They're dressed in a Star Trek-ian type outfit, [but] they were just themselves, because they knew we wouldn't see their faces [on screen]." (Star Trek Monthly issue 91, p. 32)
The writers of ENT originally planned to reveal this mysterious character's identity in the first season finale "Shockwave". During the interim between that season and the next, Brannon Braga stated that doing so "just seemed too soon and it didn't service the story." (Star Trek: Communicator issue 139, p. 29)
The name "Future Guy" became so popular that, by the end of ENT's second season, Rick Berman and Brannon Braga were referring to him as "Future Guy" in interviews, as did Paramount's press release for Season 2 finale "The Expanse".
Because the identity of this mysterious figure was being kept secret, fans meanwhile theorized that he had to be a character who was already established. They primarily suspected he was a future version of Jonathan Archer. However, posts on Internet lists and message boards suggested virtually every character, male and female, who had ever been depicted in Star Trek. (Beyond the Final Frontier, p. 363) Because Romulans hadn't appeared in the series yet, some people were hoping Future Guy would turn out to be a Romulan. In hindsight, Rick Berman and Brannon Braga once joked that another theory had been that the character's identity would actually be revealed as J.J. Abrams. ("In Conversation: Rick Berman and Brannon Braga", ENT Season 1 Blu-ray special features)
Rick Berman has stated in interviews that at no time was the identity of "Future Guy" ever developed, nor a back story created for the character.(citation needed • edit) At the 2009 VegasCon, however, Manny Coto and Brannon Braga stated that "Future Guy" was "probably going to be a Romulan" and would have tied into the Romulan War as a future Romulan, trying to "instigate things."  In the ENT Season 1 Blu-ray featurette "In Conversation: Rick Berman and Brannon Braga", Braga remarked, "We had thrown around a lot of ideas about who Future Guy was gonna be. A Romulan [for instance]."
In November 2012, Brannon Braga stated on his Twitter account that the show's fifth season would have revealed Future Guy to be Jonathan Archer "trying to correct history" and repair a corrupt future by influencing his younger self. Braga added that Archer being Future Guy was the plan from the beginning, and previous statements claiming Future Guy was intended to be a Romulan were meant as a red herring. 
In December 2012, Braga expanded on his previous statements by indicating that Archer may have been kept alive by Suliban genetics.  It was also indicated that Archer may have had a motive to even kill his past self to benefit the future. 
Brannon Braga later recalled, "I thought the most interesting idea that we came up with [regarding the identity of Future Guy] was that it was going to be Archer himself. And an Archer from some sort of terrible future where he, in some way, was responsible for doing something wrong, but he's manipulating himself. And if you go back and look, it could very well have been Archer." Braga also expressed disappointment at never having gotten a chance to establish that Future Guy was really Archer and to have Archer transform into this other persona, traveling to the 29th century (which Braga reckoned had been discussed by the makers of ENT). ("In Conversation: Rick Berman and Brannon Braga", ENT Season 1 Blu-ray special features)
The novel Watching the Clock refers to him as "Future Guy" and "the Sponsor" before revealing his name to be Jamran Harnoth, and that he is of Suliban, Romulan, and Tandaran descent, attempting to manipulate history so that he can use his civilization's genetic engineering technology to alter species across time. Watching the Clock also clarifies that his goals were actually based around manipulating the various species he attacked so that certain key temporal scientists would not be born and thus not attend a vital conference in 2381, which Harnoth believed would be the first step towards the construction of a subspace monitoring grid that was intended to alert the present to any attempts by time travelers from the future to interfere in the past. He is apprehended by Daniels and faces trial in the 31st century for his crimes.