Synopsis of why Furuichi is the greatest character ever:
OH DEAR LORD SPOILERS
In the series Beelzebub, much of the charm is character based and developed over time. We love Oga as a main character for being sadistic and stupid, regardless of how ridiculous it seems for the 'good guy'. But lets take a look at who I consider the deuteragonist of the series, Furuichi. Before receiving his powers, he's plain and wimpy. He fights with his mind, which is how he eventually gained the title tactician, and tends to Ulysses his way out of most situations. In some ways, he seems like a representation of most other manga leads: mildly pervy, weak but not weak, clever, relatable, putting the David in David & Goliath. If he wasn't the butt monkey Oga needed as motivation to get off his ass and start kicking ass, he might be a pretty solid main character.
After getting his powers, he becomes a total badass. The reveal that he resented enduring his life as the butt of every joke really struck home and made him a pitiable character with an amazing villain (?) arc. Now there's very little separating him and Oga in terms of strength; hell, they even go toe to toe for quite some time. Of course he returns to the team later, but those chapters are some of the greatest in the entire series and earned him respect from not only his friends and us readers but also the entire school district and Pillar Squad.
Thematically speaking, his powers are even crazier and more of a wild card than Oga's, who is supposed to be a wild card. Furuichi plays like it's a game of Top Trumps and is a master strategist to boot. According to the last chapter of the manga, even his personality changes from fight to fight, which is awesome. The tissues of whatever-they're-called grant him connections with powerful demons, perfect for a tactician of his caliber. He's even on friendly terms with the Pillar team through mutual respect. He managed to impress Jabberwock, for Hell's sake.
A guest by the name of Delvin Dracul also pointed this out to me, so here is a direct quote:
It's so good to see another fan who sees the greatness that is Furuichi Takayuki. He's my absolute favorite character in the entire series (and that's saying a lot considering all the good characters). You forgot to mention how he's a master at manipulation. He tricked the entire Ishiyama gang into looking for En, used the power of the Pillar Division to perfection, outsmarted Kunieda (and every other delinquent), and has even tricked Hilda! This man is a genius in every sense of the word.
In conclusion, Oga appeals to us as a character because he's both evil incarnate and an honorable delinquent, but Furuichi is neither. He doesn't fit in with the majority of the cast and is frequently left out (or wants to be left out). He represents the classic nice guy in manga and doesn't embody the same sort of craziness as the rest of his peers. He's like Nami from One Piece or Jafar from Magi, the rational foil to Oga's insanity. If this were any other manga he would probably be an all powerful harem king of his own. But it's Beelzebub, and we love Creepichi where he is, right next to Oga.
Not inside Oga, mind you. I have a very high tolerance for yaoi, butt fuck that.
