https://gamerant.com/commander-keen-cre ... ey-keen-4/
In the past, Hall has revealed that the character of Commander Keen is at least partially based on his real-life brother. Wanting to know more about the other characters, Twitter user Ian Hanschen tweeted to Hall and another Keen alum, John Romero, asking, "did one of you put your then-girlfriend into Keen 4?" in reference to the character of Princess Lindsey.
Hall replied to the tweet, explaining, "That was a Keen fan, Lindsey Velez, who wrote us a fan letter hoping for a female protagonist too!" In the same tweet he also reveals that the fire monster in Commander Keen 4 called the Berkeloid was inspired by a fan named Ryan Berke who had told the Keen team he'd seen a fire creature in a dream.