Fortress Lad is an alien from the planet Fwang. Because that planet is constantly besieged by meteor storms, all Fwangian boys, at puberty, gain the ability to transform themselves into metallic fortresses for the protection of others.
Fortress Lad attended the very first tryouts for the Legion of Superheroes, the same one where Arm-Fall-Off Boy debuted. He was very nervous but desperately wanted to fight for peace and justice alongside the other Legionnaires. Unfortunately, Fortress Lad was rejected because the Legion did not think his power would be very useful to them.
However, Fortress Lad had hung around hoping for a second shot and was near the Legion when Mnemonic Kid attacked them, temporarily stealing their memories. When Mnemonic Kid fired a ray gun at the helpless Legionnaires, Fortress Lad rescued them by transforming into a Fortress to protect the Legionnaires. This enraged Mnemonic Kid and she used her power on Fortress Lad, trying to make him forget to protect the Legion. Fortress Lad knew he had to buy the Legion time to remember their powers, and through sheer force of will forced himself to keep protecting them. He was successful, and the Legionnaires were able to to stop Mnemonic Kid. Unfortunately, Fortress Lad's memory was completely destroyed by Mnemonic Kid and he was stuck as a fortress forever, as the Legion's memories of him were also destroyed.
The Legionnaires wondered where the strange fortress had come from, but they knew that they felt some affection for it and decided to make it their new clubhouse. Years later this clubhouse became the Legion of Substitute Heroes' headquarters.
Thus, Fortress Lad got to join the Legion after all, although no one ever knew it,