When Niou corners him one day and lays out his plans for a new trick, one that he can't do by himself, Yagyuu agrees to it because he knows that Niou is very troublesome when he can't have what he wants, and because he knows that sooner or later Niou will get tired of this prank and move on to others.
It is typical of Niou to have a short, shifting attention span and to easily get bored; it is also typical of Niou to defy what is typical of him.

It comes to a point when the switch is no longer a switch, when things transcend beyond putting on another face and wearing another personality. The line between actor and role blurs into nonexistence. Eventually, Yagyuu begins to feel as though Niou is the real Yagyuu, and he wonders if Niou feels that Yagyuu is the real Niou.

"Are you Yagyuu?" he asks once. "Am I?"

He sees himself reflected in the frames of Niou's glasses, but Niou doesn't aswer. Yagyuu smashes his fist against the bathroom mirror because it is something Niou would do (and something Yagyuu wouldn't, but isn't he supposed to be Yagyuu?), and because Yagyuu doesn't like asking questions and despises not getting answers.

"Sometimes I wonder," Niou finally says, tone as crisp and immaculate as his clothes, "if we really switch when we switch, or if switching actually lets us be who we really are."

The statement raises more questions than answers.
But Yagyuu is Niou that day, so he smiles like the trickster that he is and laughs and tells Yagyuu-kun that he is thinking too much.

Later that night, when his hair is brown and tame again, he thinks, Maybe we're two halves of the same person.
Inner-Niou whispers, We are each half a person and two people at the same time.

For some ineffable reason, it makes sense.
His mind is a background of facts, and it insists that that logic is illogical, but he silences it with the comfort of knowing that there's another him living in the same world.