Leon is a memorable character for two main reasons.
The first reason is his absolutely brutal execution. As the first execution in the Danganronpa series, it actually doesn't do a great job of setting the tone; it feels much darker, more vicious and more serious than most Danganronpa executions, meaning the player will probably be surprised and confused when later executions turn out to be wackier. But it does present an impactful and unforgettable ending to the first trial.
The second reason is all his stupid facial expressions. I was genuinely disappointed that Leon went out so early, even though I wasn't particularly interested in him as a character, because his expressions were so great. I'll miss your face, Leon. I hope that's some comfort to you as you're horribly executed.