Short answers, pulled from the story.
Jeff Perren was an American game designer and hobby shop owner, best known as a co-creator of the medieval miniatures wargame Chainmail alongside Gary Gygax. He also co-designed Cavaliers and Roundheads (1973), the first game published by TSR.
Perren developed four pages of mass-combat rules for Elastolin miniature figures early in 1970, then collaborated with Gygax to expand and formalize those rules into Chainmail. The rules were first published in The Domesday Book, a fanzine for the Castle and Crusade Society.
The Lake Geneva Tactical Studies Association (LGTSA) was a miniatures wargaming group whose early members included Jeff Perren, Gary Gygax, Terry Kuntz, Rob Kuntz, Ernie Gygax, Mike Reese, Leon Tucker, and Don Kaye. The group's collaborative design work produced Chainmail and laid groundwork for later developments in tabletop gaming.
Cavaliers and Roundheads was a miniatures game designed by Perren and Gygax and published in 1973. It was the first game released by TSR, the company Gary Gygax founded, making it a landmark in the history of hobby game publishing.
The original Chainmail rules were published in The Domesday Book, a fanzine produced for the Castle and Crusade Society. This grassroots publication gave the rules early circulation among dedicated miniatures wargaming enthusiasts.
Perren and Gygax were both early members of the Lake Geneva Tactical Studies Association. Perren had already written his own rules for Siege of Bodenburg and shared them with Gygax, which led to their ongoing creative partnership on Chainmail and other projects.