Kekkaishi/WMG

'''WMG/Kekkaishi. Blatant spoilers unmarked.

Sumiko is evil
And what's she's doing now is setting herself up to be a major villain later in the story. She also was somehow responsible for killing Tokine's dad and the "Don't trust them" comment was about her. Also as hinted in the latest chapter with the "bakemono" comment she, like Byakuya is going the Transhuman Treachery route.
 * Jossed.

Sumiko is not evil, just playing her own game. Maybe literally.
And while she may not be good, she's not going to be a villain later on except maybe as an Anti-Villain.
 * Jossed.

"Sumimura Shigemori" is actually Doctor Wily
Because seriously, they look exactly the same. Obviously this universe split from the megaman universe when he wasn't chosen as the heir, and devoted his talents to robotics instead of creating barriers.

Tokimori is Karasumori's father.
Blond hair, powerful spiritual abilities, a direct interest in his well being and giving a pet as a gift is one thing. But his earnest attempt to pat the kid on the head makes me think that the two have a very sad story to tell us. If nothing else, I expect them to be related.
 * Confirmed. Yoshimori finds out and is told the story behind it by Tokimori.

The World Yoshimori Makes for Karasumori will be a Cake Castle
Honestly, this seems so obvious that I'm surprised it hasn't been mentioned here. After all, by doing this Yoshimori would achieve two of his life goals: permanently sealing away Karasumori's power, and building a cake-castle large enough to live in.
 * Busted. It's just a bigger castle. With every human and monster, alive or dead, as Karasumori's playmates, including

There is a real reason behind the feud
If you notice that the only people that ever seem to care about the feud are the owners of the Honin mark themselves, and that the rest of the family treat both sides with respect. That seems a little weird for two families that have been at logger head for 400 hundred years. So my theory is that Tokimori specifically forbade the owners of the honin mark from getting married, or that them getting married would be a bad thing, create another Karsumori etc, so the feud was made as a method to try and make the owners of the honin mark, comrades in the war to protect Karsumori, to stay as Just Friends.
 * The creating another Karsumori part is Jossed, but the feud was not started by an argument, but to make them rivals to make them be stronger so that they could protect Karsumori as silly as that sounds.

Yoshimori's grandfather takes the feud seriously because he's Tokiko's Unlucky Childhood Friend.
Yoshimori's relationship with Tokine is strongly implied to be a Generation Xerox of their grandparents. He's angry at her because she broke his heart.

The Kokuburo represent the Seven Deadly Sins
I think it goes like this:
 * The Princess is Gluttony (she hungers for power)
 * Byaku is Greed (his actions were all to solely benefit himself)
 * Kaguro is Lust (Blood Lust that is)
 * Shion is Sloth (she hates exerting energy and is generally indifferent)
 * Aihi is Envy (she envies humans abilities to show compassion; unlike a certain other Envy she doesn't wish them to suffer, instead, she wants to "take" this ability from them the only way she knows how...by replicating it)
 * Gagin is Wrath (Unstoppable Rage)
 * Either Koshu (because of his love for the castle) or Hekian (because of his condescending nature) could be Pride. The Princess might also be Pride.