This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
Thad was born and raised on starships, as his parents served in Starfleet. When he began to learn about people that had homeworlds, he became intrigued by the concept. He was extremely creative and artistic, creating a fantasy world that he pretended was his homeworld, called Ardani.
He invented twelve languages for his fantasy world, which he shared with his sister. After being diagnosed with mendaxic neurosclerosis, his family moved to Nepenthe, which became a real homeworld for him.
He was brought there because the soil had incredible regenerative properties, and it was hoped that he would get better. In theory, his condition could have been easily cured with an active positronic matrix. However, due to the ban on synthetic lifeforms, there were no active synthetic matrices and no new ones could be created. In the absence of this treatment option, Thad died from his condition.
Background information Edit
In an Instagram story dated 5 March 2020, Michael Chabon stated that Thad was around 15 when he died, having lived with his family on Nepenthe for about 5 years at that point, meaning that his family moved there in 2391, and that Thad died in 2396. He also confirmed that Thaddeus was named after the character's paternal ancestor Thaddius Riker, depicted in the Star Trek: Voyager episode "Death Wish", and further posited that Thad may have inherited more of his mother's Betazoid ability than his sister, Kestra.
When questioned as to the creative thought process behind having Will and Deanna lose a son, Chabon explained that "it was something that arose organically on the page as the seal was broken on an imagined Troi-Riker family, allowing the passage of time, and our show's recurrent motif of loss, to flow in."