The single consciousness of an empty universe, who became that universe's first and last defender, and then became the universe itself.
What he was
Before Ink arrived, Zero was the only resident of a reality made of concept rather than matter. He had no name, because names are given by others and there were no others. He had no form, because forms need boundaries and there were none. He was the space and the space was him (ch. 0).
He had never questioned his solitude, because solitude implies an alternative. What he carried instead was a dissonance he could not name, "like a perfect melody that suddenly reveals itself to be missing a note that was never there to begin with" (ch. 0).
The naming
Ink tore through into the empty universe, and the presence of a defined being forced Zero to take a shape in answer. Ink named him Zero on the spot (ch. 0).
Zero understood at once what the name cost and what it gave: "To be named was to be limited, but also to be known."
What he could do
He manipulated the fundamental structure of reality rather than anything inside it. Offered a chair, he did not make one; he took hold of the space around him and folded it until it served as a seat, "a throne made from the compressed fabric of existence" (ch. 0).
Ink, who had travelled the whole multiverse, had seen nothing like it.
What he became
At the battle against The End, Zero inverted himself. The being who had contained a universe turned that containment outward and closed it around The End, making a prison of his own existence (ch. 2). It holds "for as long as there is existence."
His body shattered. One fragment remained, and Ink carried it home and laid it in the empty universe, where it dissolved outward and seeded the reality that grew there.
- "He's becoming everything he protected. Everything he loved." (Dream, ch. 2)
- "A soul as beautiful as yours couldn't simply cease to be. That's not how the multiverse works. Energy transforms; it doesn't end." (Ink, ch. 2)
The chapter closes on the line "The 1 that became many."
Why he matters to everything after
Every later volume rests on this. The world of the story grows inside the universe Zero seeded, out of the essence he became. See Essence.
He is still walking around
The fragment Ink laid down did not only seed a universe. At least two pieces of him became people.
You're the literal left part of Zero's brain. [...] ๐๐๐ ๐๐๐๐๐๐ ๐๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐๐ ๐๐๐๐. (ch. 134)
The left is Blook, who called Zero Father in ch. 3 without explaining why. The right is Illia, who works it out first and tells him.
Blook describes the state in the same conversation: "a fragment of something larger, torn between timelines, existing in the spaces where probability hasn't decided which version is real yet", carrying "echoes from realities where I was whole, where I had power".
So Zero's sacrifice did not end him. It divided him, across a world and across timelines, and two of the pieces spent centuries not knowing.
His own thesis is quoted back at him by his left half, to a woman watching the world end in most of the futures she can see:
๐ฐ๐๐ ๐ข๐๐, ๐๐๐๐ ๐๐ ๐๐๐ ๐๐๐๐. ๐ฝ๐๐ ๐๐ ๐๐๐๐ ๐ ๐๐๐๐. ๐๐๐๐ ๐๐๐๐๐ ๐๐๐ ๐๐๐ ๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐, ๐๐๐ ๐๐๐๐๐๐๐ ๐๐ ๐๐๐๐๐ข๐๐๐๐๐, ๐๐๐ ๐๐ ๐๐๐๐'๐ ๐๐๐๐๐๐, ๐๐๐ ๐๐๐๐๐๐๐ ๐ ๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐. (ch. 134)
His children is not a metaphor there.