Doctor Altan Inigo Soong was a male Human scientist who lived in the 24th century. A self-described "mad scientist", he was the son of Noonian Soong, the creator of the Soong-type androids, and, as such, was the Human "brother" of B-4, Data, and Lore. (PIC: "Et in Arcadia Ego, Part 1")
After the ban on artificial lifeforms following the Attack on Mars in 2385, Soong and Dr. Bruce Maddox set up a laboratory on Coppelius, the fourth planet of the Ghulion system, where they continued their work despite the synth ban. (PIC: "Et in Arcadia Ego, Part 1")
However, Maddox was convinced that the Federation and other forces were actually responsible for the attack on Mars, and so he created the twin androids Soji and Dahj Asha (through fractal neuronic cloning, based on a positronic neuron recovered from Data) to discover the truth. Soong, however, believed it would result in unwanted attention, and would lead to their sanctuary being discovered. (PIC: "Et in Arcadia Ego, Part 1")
While Maddox left to set up his own lab, Soong remained behind on Coppelius with the community of androids that he and Maddox had helped create, acting as a surrogate "father" to them. He also created and owned an artificial lifeform that appeared as a cat named Spot II, based on Data's cat Spot. Additionally, Soong had created synthetic butterflies simply because he had missed seeing them. (PIC: "Et in Arcadia Ego, Part 1")
By 2399, Soong had resumed Maddox's research on transferring sentience into an artificial body called a golem, claiming a "sense of urgency". Around that time, Data's former captain, Jean-Luc Picard, had arrived on Coppelius aboard La Sirena to warn that the Zhat Vash, a group of Romulan anti-synth fanatics, were en route with an invasion fleet to destroy them. One of Soong's "daughters", Sutra, who had learned Vulcan techniques such as the mind meld, was able to discover through Maddox's former colleague (and lover) Agnes Jurati that the "Admonition" the Zhat Vash claimed as a warning against synthetics was actually a message from an ancient alliance of artificial lifeforms. The message offered protection against the "oppression" of synthetic lifeforms, even if that meant exterminating all organic life. Hoping to protect his "children", Soong worked with Sutra to build a beacon capable of summoning these protectors. (PIC: "Et in Arcadia Ego, Part 1")
As he worked to perfect his golem, Soong was also attempting to recover the memories of one of his "children", Saga, who had apparently been killed by the Zhat Vash agent Narek during his escape from Coppelius Station. When he accessed Saga's memories, however, he discovered that it was Sutra, aided by Narek, who had killed Saga. He chose to aid the crew of La Sirena in disabling the transmitter, and he himself deactivated Sutra, lamenting that she had been no better than the people trying to destroy her and her siblings. After the Romulan attack was averted, Picard succumbed to a terminal illness that had been diagnosed just before he left Earth. Before Picard's brain functions ceased, however, Soong worked with Jurati and Soji to create a map of Picard's mind and transfer his consciousness into the golem, allowing him to live what years he would have had without his illness. (PIC: "Et in Arcadia Ego, Part 2")
"I take it my face looks familiar."
"I feel as if I'm looking at Data."
"Data if he had gotten old and gone soft. Must be disturbing. I know it is for me. Doctor Altan Inigo Soong, mad scientist. My father had me, but he created Data – a fact he never let me forget."
"What you did....shame on you, Agnes. You put out a small, bright candle shedding its light in a vast darkness."
Dr. Altan Soong was played by Brent Spiner. According to showrunner Michael Chabon, Spiner came up with the "A.I." initials for the character, as a pun on "artificial intelligence".  Chabon also said Altan Soong was not Juliana's son, but a child from one of Noonien Soong's prior relationships.