King Robin was a Bruce who had chosen a different path in the Dark Multiverse.
When his parents were murdered in an alley, what he saw was not a bat, but a robin flitting wildly beneath a streetlamp, defying the laws of day and night.
So he became Robin, and his world became one without Batwoman.
He chose to spend his entire fortune to avenge his family immediately. After killing his own enemies, he turned on Gotham's criminals. Once he had wiped them all out, drunk on his own intelligence and power, he began slaughtering the Superheroes of his Earth.
This was a twelve-year-old Bruce Wayne, but his body had been enhanced by the Anti-Life Equation. He was Darkseid in a Robin uniform, perhaps with all manner of mutagenic Serums and confiscated superpowered items thrown in as well.
For example, Zeus's divine power, Lobo's Iron Chain, Nth Metal Poison Needles, Fifth-Dimensional Beings in jars, and so on. All of these tools were stored in his Little Bird Belt.
In just a few minutes, he wiped out every Superhero on his Earth and became the King of the Universe.
But it was then that The Batman Who Laughs noticed him. Across countless universes, Bruce had always chosen to become Batwoman. Who would have thought that someone would want to become a little bird instead?
And there was something even more interesting: without Batwoman, this Earth had no Joker, yet it was hundreds of times more chaotic and filthy than worlds that had both Batwoman and Joker.
So The Batman Who Laughs recruited him and invited him to plot great things together. After all, didn't every Batwoman need their own Robin?
What about the Robin Hounds? They were dogs. How could dogs be qualified to stand beside the throne in the future?