Jillas is something that The Slayers is very good at: a comic-relief turned lone wolf. Or actually, in his case, fox. He's first introduced as Gravos' sidekick, their attempt to get the sword of light providing more comedy than anything else. But with the gradual losses of Gravos and his master Valgaav, Jillas turns more and more serious. When the little guy who has made you laugh so much earlier starts to develop an unwavering determination, you can't help to root for him, even if he's misguided. It also helps that, as he gets more and more determined, his specialized skills, which previously resulted in consistent laughable fumbles, start to actually achieve their goals. What results is an ingenious and ever-resourceful character, almost reminiscent of Zelgadiss. Jillas is really what you could call the outer world equivalent of an almighty warlock from Lina's world. It makes total sense that, in the absence of strong enough magical currents in the outer world for people to develop into powerful wizards, people with smart and crafty academic background would instead gain power through science and technology. Which is what led to the development of technological advances barely known to Lina's world, such as cannons and gunpowder, at which Jillas is an expert, both as a craftsman and a marksman. Given that Zelgadiss is by far the member of the Slayers with the most affinity with devices and technology, it's a pity that a certain connection between him an Jillas didn't develop.