In Digimon Adventure 02 The Beginning, Rui is presented as the first human to come into direct contact with a digimon without any mediation from the usual system of the chosen children, a screen, or a global crisis in the human and digital worlds. I take this with a grain of salt because the chosen children aren't reliable narrators and no one fully understands the rules of such a fluctuating world as the digital one.
Ukkomon appears at a critical moment in Rui's life when he's hurt, cold and alone. It happens on his 4th birthday apparently as a result of his wish not to be alone. Its appearance introduces the idea that a digimon bond can begin from a human desire.
From that point on, what forms between them is the fulfillment of an unfiltered request, which makes Rui the origin of an anomaly within the logic of Digimon Adventure, because his connection with Ukkomon come from personal lack.
The contact takes place on February 29, 1996 when Ukkomon appears in front of him and offers to grant his wish of having many friends around the world and constant care and protection, which sets off a chain of effects where Rui's life improves artificially as his surroundings are reshaped to remove pain and absence. But that improvement is a simulation sustained by the Ukkomon, interpreting the wish as a literal command and carrying it out without understanding its consequences.
Rui says Ukkomon can grant wishes and that it's connected to a "great being", and that raises the question of what that "great being" actually is, because if it were Homeostasis as Hikari suggests, it doesn't quite fit since Homeostasis is tied to balance and Rui's situation was completely unbalanced, which supports Miyako's idea that this could be something unique to "the first" human-digimon bond.
Maybe the "great being" is something ancient that predates Homeostasis, or even Homeostasis itself using Rui as a prototype for an early test of human-digimon bonding.
When Rui realizes his happiness isn't real and that his family and surroundings are being held together by Ukkomon (possibly through its technique Liar Dream), the relationship breaks violently as Rui tries to destroy it and Ukkomon accepts the damage without resistance, until Rui screams that he never asked for any of it, that Ukkomon doesn't understand him or even know him, which causes Ukkomon to dissolve, turn into a digiegg and quickly disappear in front of him when he's 11 in 2003, the same day Omegamon was defeated by Diaboromon and Imperialdramon Paladin Mode appeared.
9 years later, on February 21, 2012, Ukkomon reappears as a giant digiegg above the Tokyo Tower, broadcasting a message across every digital screen in the world that reads:
"May everyone in the world have friends. May they each have a Digimon."
No one understands what this anomaly is until a week later Rui climbs the Tokyo Tower trying to make contact with the digiegg but fails, and is rescued by Stingmon, which leads the others to learn about his situation and decide to help him.
When Daisuke steps in to support Rui, Ken joins him and together they take Rui toward the digiegg with the help of Paildramon, but Rui is then pulled by a force into what seems like a spatial dimension where Daisuke, Ken, V-mon and Wormmon also end up. This space reveals Rui's past and shows what I mentioned at the beginning.
From my perspective, this wasn't literal time travel since the passage of time outside didn't match what they experienced inside as Miyako, Hikari, Iori, and Takeru point out, but rather a kind of spatial dimension or simulated environment meant to make Rui confront the origin of his bond with Ukkomon.
In the middle of the crisis on Rui's 20th birthday (February 29, 2012), with BigUkkomon creating billions of digieggs so every human can have a digimon, Takeru hesitates and suggests that if Ukkomon was the one who originated the bonds, then destroying it could mean losing their own, which shows that even those who have already lived through and built their relationships still carry the idea that the bond might depend on an external origin rather than on what they have built themselves.
When Rui and Ukkomon finally talk, they both realize that they never really knew each other and that what existed between them was a misinterpreted desire. From that moment on, the human-digimon bond is seen as something that exists when there's mutual recognition, a real conversation between both sides and a continuous connection between two minds/hearts that influence and understand each other (this was already quite clear in Kizuna but it's given much more emphasis here).
And that connects directly with the final resolution when Takeru says that digivices and programs aren't what bind humans and digimon, but that this connection depends on a direct relationship between the two, which corrects his earlier doubt.
"There are things that change with time. There are things that at times seem right, but there's no guarantee these won't change. By releasing its grip on important bonds, a story merely followed a journey into a predetermined future came to an end. Thus, the firm, heavy shackles were cut away, and the world took its first step toward new possibilities."
The final narration reinforces that idea, since bonds aren't fixed structures but living relationships that can change, break, or be redefined, and that's where the "new possibility" emerges (02 epilogue).
