(covers information from several alternate timelines)
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's 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 communications 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, 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")
Appearances and references
The Humanoid Figure, aka "Future Guy", was played by actor James Horan.
This character was known only as "Humanoid Figure" and was referred to as that in the episode scripts and press releases. Given the character is from the future, fans began referring to him as "Future Guy" (The name was originally a sarcastic one coined by Chuck Sonnenburg, who runs the popular SF Debris website which, among other things, reviews Star Trek episodes and movies). This name became so popular that, by the end of ENT's second season, series co-creators and executive producers Rick Berman and Brannon Braga were referring to him as Future Guy in interviews, as did Paramount's press release for "The Expanse".
Future Guy is also sometimes called "Doctor Claw" in some fan circles, after the 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.
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) However, the writers of ENT originally planned to reveal the mysterious character's identity in the first season finale "Shockwave". In 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) At the 2009 VegasCon, 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 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. 
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 civilisation'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.