BIOGRAPHY
The Stranger is an enigma, and deliberately so. He takes pride in his individuality in the universe and has a deeply solitary nature. The Stranger values his privacy to the point of lying about his origins, sometimes telling multiple, contradictory tales in succession in an effort to obfuscate any knowledge about his true nature. [Silver Surfer (3rd series) #31]
The Stranger is a scientist by nature, deeply interested in alien species and the mutations which guide and divert their paths of evolution. Seeing himself (perhaps accurately) as a higher order of lifeform, the Stranger has no qualms about absconding with sentient beings back to his Laboratory World to be used as test subjects for his experiments. The origin of Labworld is 
unknown, but it may be a fully artificial planetoid created specifically for the Stranger’s studies. His work runs the gamut from observing specimens in controlled environments to actual vivisection to see what makes them tick. He occasionally claims to be studying mutations on behalf of his people, [X-Men (1st series) #11] but most sources depict him as a singular individual with no “people” or at least no current ties to them.
According to one origin, the Stranger is the living embodiment of the people of Gigantus. In the distant past, the planet Gigantus was reportedly so large that it dwarfed entire galaxies, a Dyson Sphere encircling their star and fueling their vast civilization of billions upon billions of souls. The Gigantians were peaceful scientists under normal circumstances, relying on the sheer scale of their world to deter invasion. Still, it would become a target for the world-ravaging immortals known as the Eternals of Eyung. The war between Eyung and Gigantus led to both worlds being destroyed by anti-matter reaction bombs. In its final moments, the collective physical and psionic 
might of Eyung was concentrated into their Champion of Champions, Grom the Over-Mind. Grom’s stasis cocoon was cast out into space until he could become accustomed to his new power after eons of development. Shortly after the Over-Mind emerged in the modern era, The Stranger confronted him and announced himself to be a similar aggregate of the surviving minds of Gigantus. [Fantastic Four (1st series) #115-116]
However, the Stranger later denounced this origin. [Silver Surfer (3rd series) #31] Instead, he claimed to have found the gestating Gigantian Uni-Mind entity as one of his earliest potential test subjects. The Gigantian did not survive the Stranger’s intrusive probing, but telepathically passed on his history and mission to oppose Eyung and the prophesized rise of the Over-Mind. Apparently feeling regret for the accidental termination of the entity, the Stranger accepted responsibility for their mission as a matter of honor. When he first encountered the Over-Mind, the Stranger streamlined his intent, claiming to BE the Gigantian Uni-Mind instead of merely acting on behalf of the Gigantian Uni-Mind. [Quasar #16]
There are reasons to doubt the Stranger’s claim he had no real connection to Gigantus. He disposed of the Over-Mind rather easily when he first claimed to be Gigantian, but was struggling against him in their second encounter when he suddenly renounced his ties to Over-Mind’s eternal nemesis. There’s also his blatant willingness to lie about his past in order to seem more enigmatic to consider. In a recent story, the Stranger once again identified as the composite mind of the people of Gigantus. In fact, he actively suffered consequences as different aspects of his gestalt mind acted independently, giving him personalities with contradictory goals and demeanor, which could serve as further explanation for the Stranger’s often inconsistent behavior. Here, he claimed that the end goal of all his research in mutation and biology over the years was to create new bodies to house the souls of Gigantus that made up his aggregate. The Stranger was allegedly a living Ark designed to persist until a New Gigantus could be founded. [Silver Surfer: Ghost Light #1-5]
[Note: Originally, the war was simply the Eternals vs. the Gigantians. Quasar #16 clarified the two parties were the Eternals of Eyung and the Eternals of Gigantus. It was not explicitly confirmed that these were the Celestial-derived subspecies “Eternals,” though it seems likely. On rare but documented occasions, the Stranger is also referred to as the Gigantian Uni-Mind, like the form the Eternals of Earth can become.]
According to one source, the Stranger was a member of the Elders of the Universe. These beings are each the sole surviving member of their individual race, among the earliest civilizations to rise and apparently fall in the universe. Their immortality is derived from tapping into the Power Primordial and fueled by their monomaniacal devotion to a particular pursuit. There are reasonable comparisons which can be drawn between the Stranger and the Elders, and his solitary nature could excuse his absence from most of their gatherings. However, the one time he appeared among the Elders, they also included the In-Betweener, a being known to have very different origins. The Elders have been known to “fudge” their membership requirements in the past, such as when they convinced Ego to join their number. [Avengers Assemble #7-8] Furthermore, the Elders were presented here akin to the abtract entities like Death and Eternity, instead of as merely extremely powerful immortals. This manifestation was highlighted as dubious by a time-traveling Starfox and Pip, who theorized their haphazard distortion of the timestream in this aborted timeline may have led to the Stranger and the Elders appearing in this unusual manner. (It's cosmic. It's funky.) [Thanos: The Infinity Ending]
Additionally, even one further level above that, the Stranger has been depicted on the scale of the abstract entities who represent the fundamental concepts of the universe. The Stranger made repeat appearances in the court of the Living Tribunal, standing alongside Galactus, Eternity, Death, the Celestials and others as one of the highest powers in existence. [Infinity Gauntlet (1st 
series) #3] The Living Tribunal is the judge of all reality, the chosen representative of the One-Above-All, able to lay down judgments even against Eternity himself. To maintain the balance of the multiverse, the Tribunal weighs decisions between its three faces, representing necessity, vengeance and equity. The Living Tribunal once claimed that, if he had a fourth face, that face would be the Stranger. The Stranger, of course, refused to be defined this way. [Silver Surfer (3rd series) #31]
Oddly enough, it remains feasible for all of these contradictory origins to make sense together. The Stranger could be a scientist and the last survivor of a long dead alien race. He could literally or essentially meet the qualifications for an Elder of the Universe as a result. He could have come across the gestating mental collective of Gigantus as he claimed and, rather than merely taking an oath to fulfill its mission, may have tangibly absorbed the collective into his being, preserving it and explaining its influence on him. Mortal or terrestrial beings have ascended to the level of abstract entities in the past, such as Kronos or Maelstrom. So, the scientist who survived to become an Elder and absorbed the gestalt of Gigantus may then have assumed the aspect of the eternally unknown and unknowable independent face of the Living Tribunal known only as… the Stranger.
The Stranger has been active for eons, conducting his experiments and wandering in search of new and interesting phenomena. One of his earliest reported experiments was the creation of Ego, the Living Planet. Unable to accomplish this feat alone, the Stranger found it necessary to borrow equipment from the Elder of the Universe known as the Collector. The Elder agreed, but 
only if the Stranger created two of these sentient bio-spheres and left him one for his collection. The Stranger agreed, for he saw the possibility for future research regarding Ego and Alter-Ego, one roaming free and one raised perpetually in isolation. [Astonishing Thor #3] It should be noted that, like the Stranger, Ego has had multiple origins over the years, such as evolving from the scientist named Egros or being birthed fully formed from the living bio-verse called the Black Galaxy. [Thor (1st series) #133, 228] The Stranger claimed he deliberately spread false rumors of Ego’s origins to hide his involvement, but the reverse could just as easily be true. One subsequent account mixes tales, telling how the Stranger triggered the stellar explosion which led to Egros becoming Ego. [Ultimates 2 (2nd series) #8]