Upon discovering Earth as a habitable planet of moderate climate and unchanging day length, the alien civilization Trisolaris immediately plans on invading Earth, eager to leave the harsh and volatile triple star system they call home. However, their preliminary study has shown that mankind's technological development far outpaces theirs, and while Trisolaris still holds technological supremacy at the present stage, during the flight of the invasion fleet (which would take 400 years to reach Earth), human technology would have surpassed Trisolaris, which would spell certain doom for the fleet. Thus, the Sophon project is launched as a countermeasure to this: a lock that bolts down Earth's technological progress until the fleet arrives on Earth.
The creation of the Sophon involves "unfolding" a proton's extra dimensions, turning it into a sheet the width of a planet. Circuits are etched onto the sheet using strong interaction force before the sheet is folded back into a proton (now a quantum computer). Because of its nature as a subatomic particle, it can be accelerated to light-speed and arrive in the Solar System in advance, carrying out it's purpose: to sabotage Eath's technological development.
Upon reaching Earth, the Sophon scrambles every particle accelerators, forcing them to give false and inaccurate results each time a collision experiment is conducted. Aside from this main task, the Sophon also is responsible for spying on mankind, and getting human traitors on Earth in contact with the Trisolaran fleet. It is not until the traitors' base are raided by government forces does the world know the presence of the Sophon. The moment globel leaders and military realize this, the Sophon projects the words "You are but bugs" onto their retinae to rub in their face the fact that mankind has already lost all hope when the Sophon comes into place.
The Dark Forest
Because of the Sophon's ability to eavesdrop on conversations and read documents and electronic data, Earth's intelligence is practically laid bare and there's no more secret save one thing: human thoughts. To exploit this only weakness, the United Nations has implemented the Wallfacer project: four intellectuals that specialize in different fields, the wallfacers, are granted military power and the privilege to order any strategic movement without having to explain themselves. The project is launched in hope of outwitting the Sophon and, by extension, the Trisolarans. In response, the Trisolarans contact with the remaining human traitors in hiding, assigning four of them as the "wallbreakers". They are provided live feed of their respective wallfacer via the Sophon, and are responsible for deciphering the wallfacers' hidden motives and purposes by observation and deduction.
Out of the four wallfacers, only Luo Ji remains active to the last phase of the project. Luo Ji is a sociology professor that was once taught by Ye Wenjie, the very person that leaks Earth's coordinates to Trisolaris out of misanthropic despair, setting into motion the whole plot.
Applying Ye Wenjie's teachings to the big picture, Luo Ji deducts the nature of the "Dark Forest" that is the universe and its sapient denizens after a epiphany. To confirm his hypothesis, he broadcasts the coordinates of a unremarkable star into deep space using electromagnetic waves and the Sun's amplification properties. Later, the star is eliminated by an unknown outside force, and Luo Ji's theory is verified. However, the Trisolarans, aware of this, sends in a Droplet probe, which totally blocks off the Sun's broadcasting abilties, forcing Luo Ji into a corner.
However, Luo Ji managed to work a way out of the hopeless situation. By feigning depression and carrying out seemingly meaningless tasks, he secretly deploys H-bombs around the Sun which, upon detonation, would create dust clouds in the path of sunlight, broadcasting the coordinates of Trisolaris using visible light instead of the wave bands sealed off by the Droplet. Using this as a potent leverage, Luo Ji successfully threatens the Trisolaran fleet into aborting invasion and change course. He also negotiates the providing of technological support from Trisolaris, and also the lifting of the Sophon's sabotage.
50 years after the events of The Dark Forest, mankind is still under the truce with Trisolaris, and the Sophon is repurposed as an ambassador of Trisolaris on Earth. Instead of the practically intangible true form of its, it now inhabits the robot body in the form of a gorgeous Japanese woman who appears to be all fragile and harmless. At this time, Luo Ji has long occupied the position of the Sword-holder (the one in charge of the broadcasting of Trisolaris' coordinates into deep space), and his retirement is imminent. The would-be female Sword-holder, Cheng Xin, pays a visit to the Sophon before attending the transference, during which the Sophon says that they're both women, and are thus responsible for making the world a prosperous and beautiful place.
Cheng Xin then assumes the position of the Sword-holder, while an elderly Luo Ji finally lets down the burden after his half-century-long career as the guardian of mankind. However, a mere ten minutes after the transference, the Droplets attack Earth and wrecks all its gravitational wave emitter that are vital to broadcasting the coordinates. The Sophon reveals its true colors, sneering at a frightened and helpless Cheng Xin and telling her that it has been waiting all these years for humans to lower their guard, grow weak, and elect a gutless Sword-holder like Cheng Xin who won't swat a fly, let alone destroy a civilization (and risking her own, since Earth is very close to Trisolaris and is likely to be caught in the destruction of it). Upon sealing mankind's only means to deter Trisolaris, the Sophon acts as a herald of the second invasion of the Trisolarans, ushering all human beings on Earth to migrate to Australia disarmed, where they would be treated like indigenous people in a reserve. It uses the Droplets to enforce its words, and kills off rebellious and disobedient humans with them.
As the mankind faces impeding doom, hope comes in the form of a human spacecraft long left the Solar System during the devastation of the first Droplet. Carrying their own gravitational wave emitter on board, they broadcasts the coordinates of Trisolaris in hope of saving their fellow people some time. An year after the broadcasting, the Sophon senses the gravitational wave; knowing that there's no good invading Earth now, it beckons the second fleet to change course, as Trisolaris and Earth are both danger zone now, and are likely to be destroyed at any time.
Before departure to the second fleet, the Sophon arranges a meeting between Cheng Xin and Yun Tianming, a friend of Cheng Xin's who volunteered two centuries ago to have his brain put in a probe and sent to the first Trisolaran fleet in hope of a slim chance to gather intelligence about them. Yun Tianming, now again embodied, tells Cheng Xin three fairy tales composed by him in which he hides the secret of evading the destruction of the Solar System. Successfully passing the intel to Cheng Xin undetected by the Sophon, Yun Tianming bids her farewell through the feed, and the Sophon's robot body self-destructs shortly after, its proton form leaving the Solar System.
The next and last appearance of the Sophon is long after the destruction of the Solar System, due to mankind failing to decipher Yun Tianming's intel and due to Cheng Xin being caught in a time dilation event. Accompanied only by a human male named Guan Yifan, Cheng Xin arrives on an extraterrestrial planet as Yun Tianming instructed her to, where she and Guan find a pocket dimension. The Sophon awaits them inside (in robot form), stating that the pocket dimension is a gift from Yun Tianming, and it is now responsible for taking care of the two remaining humans. The trio survive the end of the universe by hiding in the pocket dimension. After the rebirth of the universe, the trio, led by the Sophon, leave the pocket dimension in a rudimentary spacecraft to explore the unknown.
When first created, the Sophon acts in a robotic manner in front of the Trisolaran scientists creating it, drawing a similarity between it and stereotypical depictions of A.I.s. It shows little to no autonomy during its mission on Earth, acting purely as a mouthpiece of its Trisolaran controllers, relaying messages between the fleet and human insurgents that support the invasion and the destruction of mankind.
However, during Death's End it has taken on a personality surprisingly akin to a human's. It appears all feminine, tender, and soft-spoken at first, but cold and sadistic after the reveal of its deception. It has shown apparent disgust to the lack of discipline in human refugees in Australia, to the point that it hacks several of them to death to establish order. It is also contemptuous to mankind that has grown all weak and woman-like under Luo Ji's protection. However, it does show genuine respect, even awe, at Luo Ji's vigilance and dauntlessness. Another person it holds in high regard is Thomas Wade, the Sword-holder candidate who is a ruthless and dangerous individual.
It has not shown any sign of shock or grief after the destruction of Trisolaris, and remains passive and calm instead.
The Sophon, strangely, seems to respect Yun Tianming's will, whose current status in the Trisolaran fleet is unknown, and does his bidding to a certain degree. It even honors Yun Tianming's wish long after his demise, never causing damage to Cheng Xin and looking after her after the end of human civilization.
Powers and Abilities
As the creation of a very advanced alien race, the Sophon has a wide array of powers at its disposal.
Particle Accelerator Disruption: the very task the Sophon is built to carry out. Because of its nature as a proton, the Sophon can detect the activation of any particle accelerator with the Solar System, and flits to it in no time. It would then substitute the target particle to be smashed, forcing the accelerator to yield chaotic and useless results.
Self-Restoration: after being destroyed, it can use a built-in software to piece itself back within a fraction of a second. This also makes it practically indestructible in its base proton form.
Faster-than-Light Communication: there are more than one Sophon units, each of them but a proton. Information can be relayed between the units using quantum entanglement, enabling zero-delay communication between Earth and the invasion fleet despite the the vast distance in-between.
Nigh-Omnipresent Surveillance: the Sophon can comprehend human language, and its agility allows it to spy on multiple factions on Earth at the same time. Every piece of information but thoughts are transparent in front of it, including printed document, virtual data stored on electronic devices, radio waves and spoken words.
Immense Computing Power: as a supercomputer built by an advanced civilization, the Sophon's computing power far exceeds any machine known to man.
Message Projection: by rapidly weaving in and out of a human eye, the Sophon is capable of tracing luminescent, phantom-like messages directly onto the retina, which is its main (and preferred) method of communicating with humans.
Control over the Droplets: the Sophon is in charge of the Droplets: smooth, quicksilver-like spheres the size of a small truck each. They're covered in a shell of Strong Interaction Material: a substance whose particles are bonded together not by electromagnetism, but strong interaction force, making every naturally occurring substance (that is, everything in the Solar System) as fragile as wet tissue paper in front of it. The propulsion drive of the Droplets also allow them to fly at up to 30% light-speed, and their toughness enables them to perform acute turns while operating at third cosmic velocity without pulverizing themselves. Although the Droplets actually serve as space probes rather than weapons, they alone are more than capable of annihilating the entirety of Earth's space forces.
Japanese Sword Proficiency: in robot form, the Sophon can wield two Japanese swords with deadly finesse. It can swing them faster than human eyes can follow, intercept and cut tossed stones mid-air, and slice a human in half with a callous slash.