Catch up with the first installment looking at "Kuroko Shirai"
Catch up with the second installment looking at "Accelerator"
Catch up with the third installment looking at "Takitsubo Rikou"
Catch up with the fourth installment looking at "Mugino Shizuri"
Catch up with the fifth installment looking at "Last Order"
Also a priest, Magnus is part of Necessarius (or the Church of Neccesary Evil) and the first 'evil' magician we met in season one. When he's not teamed up with Kaori Kanzaki, he sometimes takes on the role of Touma's partner/handler/keeper when the situation calls for it or when the Church specifically orders the team up, much to the jargon of both parties. It's a bit of a love-hate relationship. There is a mutual respect buried in there somewhere as well, but mostly it's just comes down to them annoying one another with their clashing ways of dealing with things.
Stiyl's is quite the opponent, whose fire magic at first might seem as though it's basically just the magical equivalent to any Pyrokinetic Esper living in Academy City. He does however showcase that he's not just a magical knock-off with his signature move - Innocentius - a summoning spell that uses rune cards to call forth a fire demon that burns at 3000 degrees.
Innocentius itself has several unique powers of it's own. The first prevents it from leaving the battle unless all the rune cards that were used to summon it are neutralized or destroyed. It also comes with a self-regeneration power which is a bitch to deal with regardless of franchise. Monsters with regeneration abilities are game breakers in fiction and the original casters of the 'Are you f**king kidding me?' spell among audiences.
I have no idea what Stiyl's role in Index II is going to be, but I certainly enjoyed the bromance between him and Touma in season 1, so here's hoping for more of that. Although I can't say Stiyl's is my favorite character in the franchise, he's nowhere close to being my least favorite either. Come to think of it, I don't think I have a least favorite character in the franchise. Something for me to think about.