Jack Rakan
Also known as: "Rakan of the Thousand Swords"
Jack Rakan is the tall, dark-skinned fighter in Negi's group and one of his most powerful allies. He's known for pulling off genuinely devastating attacks without breaking a sweat—the guy's combat ability is legitimately impressive.
His pactio card lists him as Jacobus Racan (they swap the "k" for a "c," standard practice on all pactio cards). His artifact is a shapeshifting blade that does pretty much whatever he needs it to do, from standard sword work to summoning a massive blade that falls from the sky for one-hit-kill levels of damage. He's also shown he can fire high-powered lasers from his body whenever he feels like it.
Rakan first meets Negi in the Magical World during the tournament arc. Impressed but unimpressed (if that makes sense), he notices Negi lacks a serious ultimate technique and offers to teach him one—for a hefty price. That deal goes nowhere fast when a shadow-using assassin with a grudge against Nagi shows up. Forced to bail Negi out, Rakan has a change of heart and agrees to train him for free instead.
What Rakan teaches him is Magia Erebea, a forbidden technique that was originally created by Evangeline A.K. McDowell.
Content compiled by AnimeList.moe from publicly available sources.