Sometimes, he didn't always summon the Persona he'd been intending to.
Sometimes, certain Persona just overrode whatever he'd originally been trying to summon.
Sometimes, Yu noted, the Persona would talk to him.
The first time it happened had been after a fusion while taking on Hablerie in his and Yosuke's first attempt to get through the dungeon Yukiko's Shadow had made. It had been an accident, a mere instinct guided by the residents of the Velvet Room, as he desperately tried to think of a way to keep his friend from being strangled to death.
"Jack Lantern!" he had cried, was sure of it. And the Persona had showed up on command, at least briefly, but a sharp stab of pain had shot through his head, crippling him enough to force him down onto his knees. Then out came another Persona, one he was very sure he didn't know nor have. Very human-looking boy, very much similar to his own age, and very-much carrying a very large scythe. Blond hair, amber eyes, white wings, and an odd-looking outfit, his immediate thought was that this was just a Persona.
Except that he didn't know which Persona this was, nor any of his abilities.
"Y-You have more than one Persona?!" Yosuke exclaimed, but Yu ignored him, focusing instead on the Persona he knew nothing about. The Persona now looking right at him.
The Persona sighed, smirking a little, shouldering his scythe.
"Got yerself in some kinda trouble, huh? Are you related to Alex or somethin'?"
Yu was sure he must have looked a bit comical, given that the Persona was now laughing at him. And then his mind reminded him that, from what he'd noticed, Persona didn't talk. Except this one did.
And now it—he was flying off after the Hablerie, slashing them to pieces or just plain summoning ice wolves — ice wolves, what! — that quickly tore apart whatever was left. Vicious creatures that were clearly being controlled by the Persona and not him.
The Persona, strange boy, whatever he was, exhaled before straightening up and folding his wings, looking back at Yu, shouldering that scythe again.
"Well, I gotta get going. See ya!"
Just like that he was gone in a show of blue light, and Yu found his body giving way to exhaustion.
"Well, this is certainly unexpected."
When he opened his eyes, he wasn't with friends anymore, but in the Velvet room. There was Igor and Margaret, sitting in front of him, like usual.
"Very interesting," Igor mused, wearing his trademark grin. He just knew the man was very excited. "To think, you would have the potential to do something like this. Why, I didn't even think it was possible! It looks like this is an unforeseen difficulty in your journey."
"What? What do you mean?" he found himself asking.
"You didn't summon a Persona," Margaret informed. "Not a technically, anyway." She waves a hand nonchalantly beside her, and it's only just now that Yu is noticing that the being in question has been sitting there, arms crossed and looking a mixture of extreme boredom and complete confusion. Before he can ask questions, though, she continues. "There's been a rift. Your time is intersecting with another, and it's having an... interesting effect.
"Given the right conditions, you are able to summon these unique individuals as temporary Persona. Their hearts resonate with your own and will provide you with the strength to overcome just about any trials."
"W-Wait!" The blond boy spoke up finally, seeming startled now. "So it won't just be me?"
"Most likely not," Margaret shrugged. Now the boy was squirming though.
"Is something wrong?" Yu asked, getting the better of himself.
"J-Just... " The boy bit his lip, uncomfortable in his seat. "Ah, crap, he really wouldn't be able to handle this at all... "
"Who?"
"My... friend. He's—look, you'll know 'im when you see 'im, and when you do, just... take it easy, 'kay?"
"Sure... " Even though he didn't even know if that was something he could even do, let alone if it was something that would actually happen.
The boy grinned, looking exceptionally pleased with someone he just met. "I'll help ya out as long as I can, then." And Yu found himself smiling back. Why, he didn't know.
"Now," Igor spoke, reminding him just where he was, "I believe it's time for you to go. Your body should have recovered enough."
His vision faded to black.
"... ami... ! Narukami, get a hold of yourself!"
The first thing he noticed was that he was almost sure he'd been whacked on the head by a basalt. The second thing he noticed was that he was apparently being held up by someone. He blinked his eyes open, slowly lifting his head to see Chie's face in front of him, Yosuke in his peripheral. That quickly solved the question of who was helping him.
"Thank goodness! We thought you might've gotten hurt or something," Chie sighed with relief.
"Sensei, that was amazing! You can summon more than one Persona!" Teddy exclaimed excitedly. "You're so much better than Yosuke!"
"Hey, shut up, you!" the brunette in question snapped. His demeanor quickly changed when Yu pulled away from his friend, able to stand on his own. "Still, you freaked us out there. You okay?"
"Y-Yeah. Just... tired, I guess... " The silverette shook his head again, a vague attempt at warding off his dizziness.
After wandering around more, up some more floors of the castle, they decided to call it quits when Chie's Shadow decided to join in on the fun.
"Do you have a name?"
"Name? Oh, yeah! It's Shin."
"Just Shin?"
"Yup. You?"
"Yu. Narukami Yu."