To the northeast of Ariake is Shiba, represented by the Tokyo Tower, and Roppongi to the west.

Shiba features a Jakyou Manor and a Kaifuku, so make use of them if you need. Also, make sure you visit the Terminal at the north of town.
When you're ready, head to Tokyo Tower. Orthros will break the seal, and you'll be asked if you want to go inside.
On the second floor, a man will appear.

Man:
"Are you the saviours?"

If you say no, he'll tell you to go see "the princess" to have your wounds healed. If you say yes:

Man:
"So it's you who are after the princess!
I won't let you! Get lost!"

Eventually, you'll come across the witch you've heard so much about.

Witch:
"Welcome, I've been waiting for you."

Orthros will suddenly roar out:

Orthros:
"Yuki, you traitor!
This time won't end up like the last!
I've got the Saviours with me now!"
Yuki:
"So you've come for another lesson, Orthros!
...You two, did you let this dog fool you?
Well, that's too bad...
By the way...
If you've got something to say, put your weapons away first."

Say yes, otherwise you'll be chased away.

Yuki
"Are you the people calling themselves the Saviours?"

Answer no:

Yuki:
"Don't mess around! You know that's what everyone thinks!"

The rest follows as if you answered "yes":

Yuki:
"You must be very obedient to do something like believe Pazuzu.
He's just using you to beat Maou Bael, you know.
In other words, it's just a simple power struggle.
Well? Don't you think so?"

Answer yes:

Yuki:
"That's right!
If that's what you think, cut your ties with Pazuzu now!"

Answer yes, and Shiro will interrupt:

Shiro:
"Hold it, Tatsuya...
Who do you think we have to thank for making it this far?
This woman's just trying to fool us!"

Yuki:
"You won't believe me?"

Shiro:
"As if! Pazuzu said that you were a traitor!"

Yuki:
"You really trust Pazuzu completely.
There's no more point in talking to you...
What about you? Do you believe me?"

Answer yes, and Shiro will interrupt again:

Shiro:
"Tatsuya! Don't let her fool you, believe me!"

Refuse Shiro, and he'll say:

Shiro:
"I can't believe you'd choose the woman, Tatsuya!
Next time we meet, if you're my enemy...
Then, I'll...
I'll...
Anyway, I'll take my share of the Makka and Solomon's Ring.
And Orthros too...
Later, Tatsuya..."

And he'll run off.

Yuki:
"I'm sorry about your friend.
But, I'll fight for you in his place...
And bring back peace in the world.
If we do, I'm sure he'll understand sometime."

With that, Yuki will be your new partner.
However, if you answer no to Yuki during the conversation, she'll run off and Shiro will remain your partner instead. She won't reappear if you go back to her room, so consider this an important decision.