For those unfamiliar with the anime, one slip-up at the end (next-to-last episode) reveals Opacho to be a boy in the anime, although I believe she's a girl in the manga. The Tome of the Shaman is the Ultra Senji Ryakketsu.
Yoh
"How can they be beating you?"
Of all the answers Zeke could have given... of all the answers he should have given, Yoh would never have expected this one. It was a sensitive question, maybe. But Opacho had the right to ask it. The two are so close Yoh can't remember a time he saw Zeke with Opacho standing more than three feet away from him. And if it's obvious Zeke didn't care about his followers, he's always thought Opacho was different.
And when Opacho says the word friends before running away, he knows he wasn't the only one who thought that.
He's been inside Zeke, almost a part of his soul. Just minutes ago he was much too close to being Zeke. Inside... Zeke everywhere, surrounding him. Thoughts, feelings, opinions, intentions. Some terrifying, some understandable. Most of them were both. Like when they read the book: Zeke's intentions were dark, but his reasons were logical and even good. And if it is true that no-one wanted the world Zeke dreams of, it's still a solution. A drastic solution that Yoh would never consider, but he understands.
He understands everything about Zeke now. And he remembers how Zeke feels about Opacho. The little boy he saved from starvation, and then adopted and raised into a powerful shaman. A useful tool, but more than that. The loyalty, the trust, the affection Opacho carries for him has earned something in return.
Zeke looks after Opacho, watching the boy's short legs carry him away until he's gone, disappeared. His eyes are a fraction too wide for a moment. Is he surprised at the abandonment? No, he threatened Opacho. He must have been expecting it. It's the word friends that has him shocked like this.
He turns away, his eyes empty. He could almost be sad. He closes his eyes and leans his head forward, and when his shoulders start shaking, Yoh thinks he's crying. He can feel his friends' surprise behind him, feel their eyes boring into his back, waiting for his command. They'll strike the moment he signals them it's allowed, and yet he knows they all feel what he's feeling. That this moment can't be shattered by force, that an attack now would be treacherous.
And then... Zeke lifts his head, opens his eyes and bursts out into the most terrible laugh Yoh has ever heard. An insane laugh that sends shivers up his spine. It lasts too long, and it sounds too strange. A forced laugh, Yoh realizes.
And when the horrible laugh ends and Zeke speaks, Yoh can hardly hear him. He's focused on something. Wonders if what's glinting in the corner of his twin's eye could be...
"Poor Opacho," Zeke says, almost calmly. "Now he's going to miss all the fun."
A tear.