Hero
There are heroes of every caliber and motivation. Heroes of the devil, and heroes of the gods. And then there's Mello: once a hero, now a villain. Mello doesn't realize it, too busy chasing Near and Kira to bother, but Matt observes for him. Matt would follow him to the end -- however bitter that end may be -- and never regret it, because while Mello might not be a good example of all that is Good (on the contrary, Mello encompasses all that is Bad), Mello is far too interesting and far too addictive for Matt to ever let go.
It's not that Matt minds. Matt enjoys Mello's mood swings, and Mello's abrupt changes in plan. Even in his darkest hour, Mello is appealing (like fire -- so very dangerous, but so very intoxicating). Matt lives for Mello and those smirks late at night (when all is dark and Mello's fingers thread themselves in Matt's hair, tugging and twisting), because being with Mello is better than being with any ordinary hero.