Disclaimer: I don't own Bleach.
It was a little known fact that Shinji didn't actually fight his Inner Hollow. He suppressed it plenty, held it back of course, but he never actually engaged it in combat. When the time came for them to go head to head, Shinji slipped into his inner world and waited for the Hollow to materialize. When it did, he could only think that it was a grotesque mockery of him.
Where Shinji's hair was gold and straight, this creature's was completely white, and hung in ragged tangles around a pale face. The eyes were unusual - the sclera was pitch black, and the iris a tantalizing amber as opposed to Shinji's own pale blue. It's shinigami robes and captain's haori were in colours exactly opposite of the pieces Shinji wore - that is, where Shinji's were white, the Hollow's were black, and vice versa. It even clutched a reversed replica of Shinji's sword.
The real Shinji initiated the conversation, beginning with a long, entirely fabricated speech about how they would be stronger together. The replica sneered and laughed, before he suddenly swept his sword up, obviously meaning to plunge the tip into Shinji's neck. He clearly didn't expect Shinji to block it easily with one hand, and grin disconcertingly, even with blood dripping down his forearm.
Suddenly the Hollow grinned back, lips pulling apart to reveal too-large, too-sharp teeth identical to Shinji's. Shinji reached a hand out and grabbed his Hollow's sleeve, the grin fading from his face as he looked at his Hollow. Their eyes locked, and the grin returned to Shinji, only this time it was nasty.
He spoke again then, but this time there were no pretty words or double entendres, only a simple, closing statement which could have been anything from a calm request to a rule set in stone.
'Follow my lead.'
And the Hollow nodded.