The AiPT folks did a Cyclops Week and interviewed Fabian Nicieza where he talked about Adam-X. You can read the whole thing by click this link! But I wanted to capture the part that Fabian had mentioned about Adam:
AiPT!: Speaking of Summers family revelations, you co-created the character Adam-X with the intention of making him the third Summers brother. Do you remember where this idea to give Cyclops and Havok a half-alien half-brother came from?
Nicieza: It came kind of out of nowhere while I was scripting issue X-Men #23. I wanted to make Sinister live up to his name, so let him be a suspicious, malevolent trickster. A “slip of the lip” to either reveal a truth or send an opponent to chase after a lie made sense to me. I hadn’t necessarily thought of Adam-X or a backstory at that point, but when Bob Harras reacted positively to the idea, I immediately said, “The child of Kate Summers and D’Ken. The Shi’ar wanted to experiment with a human/Shi’ar hybrid and fertilized an egg outside of her womb and grew it in a lab. She never knew about it, Corsair never knew about it, but he’s still alive.”
Bob liked that, too, and we decided to introduce him in the X-Force Annual that was going to debut new characters.
AiPT!: Marvel eventually revealed the third Summers brother to be Gabriel Summers, or Vulcan, whose origin definitely shares similarities with what you had planned for Adam-X. How did you feel about Marvel’s resolution to what ended up becoming one of the biggest X-Men mysteries?
Nicieza: Never read it. Don’t think I ever will. That tells you all you need to know about how I feel about that. Sometimes, the lack of respect that can be shown by subsequent editorial and creative decision-makers resonates with some things more than others.