I'll start us off! Since there's nobody above me, I'll do the character below me.
You run into Dann while trying to complete a mission. If you've gone down the wrong route, you might spring one of his traps, and your mission is delayed further. While watching you fumble about, he begins to laugh hysterically. After successfully escaping his traps a few times, you'll be able to approach him to buy some of his contraptions.