A/N: Yep, I'm back at work on this story too, and without forgetting what I was doing with it either.
If you can figure out who the stranger is in this chapter, then you're may well have figured out more of the relationship between Esan and Sora - beyond what's already mentioned, that is.

Island of the Third Echo

The wall of darkness had continued its slow, steady advance while they had been on the previous island, but the moment they passed through the gate to the third it had shot forward to engulf it with a massive crash and rumble.
The waters had been pushed forward by it in a great tide, washing over the shores of the third island in a slightly less impressive crash. When it faded, the roar of water filled their ears.
"Hollow Bastion," Sora identified the echo that had formed. "The Rising Falls that surrounded it when Maleficent was trying to unlock the Final Keyhole."
"Oddly appropriate," Mickey remarked. "Since we'll probably have to go to the real thing if we can't stop the Collective."
"I don't intend to give them time to collect the princesses," Sora replied. "Sand should be back shortly; he'll give us a status update. Until then, we might as well push forward. I know the way through this area."
Mickey nodded, following his lead from floating cliff to rock, rock to ledge. He had it somewhat easier than Sora with an agility that was not readily apparent otherwise.
The island as it had appeared, like the previous two, had been destroyed. There was nothing above ground level. However, as they rose up through the area, it was apparent that either the echoes could become solid, or they remained at ground level and it was the echoes that sank down instead.
Sora paused before leaping to one of the larger, more intact chunks. Small channels filtered water around what may once have been a central square, pillars on one side.
"Something up?" Mickey asked him.
He nodded slowly, glancing down to his Keyblade a moment. "This is where I finally caught up with Riku on my first adventure. He stood opposite me on that chunk and took the Keyblade from me."
"He took it from you?"
"Yeah. We talked about it later, it was because for that moment, his Heart was stronger than mine. Donald and Goofy went with him when he left, leaving me alone with a toy sword I used to have. It wasn't until I confronted him up in the actual castle itself I got it back."
"You're worried it'll happen again, aren't you?"
"What? No, it's just a memory that came up... and a place I kinda hadn't expected to see again."
"It's just an echo, Sora," Mickey reminded him. "Come on. Let's keep moving."
Sora nodded, jumping up onto the platform, then pausing again.
"Someone's here. I can feel it."
"Who are you meant to be?" a voice asked from opposite him, where Riku had stood. He looked a lot like Riku had while using the darkness, but the suit worn was made from blacks and reds, and the stranger wore a kind of black mask that obscured his face.
In one hand was a massive weapon, possibly a Keyblade but it was difficult to tell. Like the suit, it too was black and red, with a wide handle that seemed to have a blue eye where it connected to the blade, which in turn had chains wrapped around it and a gear in the middle. The tip also resembled a gear, again connected with another blue eye.
"Vanitas!" Mickey identified him, surprised.
"How do you know me?"
"Don't you remember me? I stopped you from bullying Ven!"
"Ven... you mean Ventus. I see. I don't know those events. The question reminds... who're you meant to be?"
"You're talking to King Mickey," Sora gestured beside him, then to himself, "And Grandmaster Sora."
"Am I supposed to be impressed?" Vanitas replied. He tapped the side of mask, which retracted down to just a kind of metal guard that ran along his jaw. The face revealed looked similar to Sora's but with jet black hair, and golden eyes.
"Don't be put off," Mickey warned him when Sora drew in a sharp breath. "He's just an echo."
"An echo, hm?" Vanitas mused. "Is that what you believe?"
Mickey glanced to Sora curiously.
"I can try," he answered the unspoken question. "Without Sand there's no way to be certain though."
"It'll have to do. Give it a go."
He nodded again, closing his eyes in concentration while Mickey watched Vanitas carefully in case he made any move.
Sora drew on the power from the Arcology, sending it out in pulses around him similar to a radar system.
The echoes returned only faint pulses, as if they'd hit something further away, while anything real came back stronger. Hollow Bastion and it's rising falls were echoes, but the pulses revealed the true landscape underneath them. They were still at ground level, standing not far from the gate that lead to the fourth island.
He picked up two people nearby, one of course was Mickey.
"He's real," Sora said at last. "Not an echo. But he's not what he appears to be. What I'm getting back doesn't match what we see."
The Vanitas image nodded. "So it's true," he murmured. "Welcome home, Sora."
"What are you talking about? My home... was the Destiny Islands, and is now my Arcology."
"Home has different meanings to different people though. To our people, it will always be the Echo Islands."
"I'm not one of your people."
"You are, though you do not realise it."
"Who are you? Really, I mean."
"Esan will explain it to you when you reach him. Just tell him..." he sighed, looking over to the waterfalls, then shook his head and continued, "Tell him I'm sorry for what I did to his mother, and doubly sorry I didn't say that sooner."
"You can tell him yourself if you come with us."
"No. I know what's to come. I'll remain here and hold off the darkness for as long as possible. You'll need it when you reach the Island of the Final Echo. Now go, Sora. Go and meet Esan."
"We will, just as soon as the echo fades," Mickey replied.
"Oh. Right. I guess I forgot." He made a vague gesture with the weapon, and the ruined island returned around them. In place of the image of Vanitas was a tall yet distinguished old man that held similar features to both Sora and Roxas, but with grey hair that reached down his back, between two large, bright white wings. In place of the weapon he had been holding while appearing to be Vanitas was what appeared to be a Kingdom Key, burnished the same white as his wings. Unlike the Kingdom Keys Sora knew of so far, this one had teeth on both sides of the blade, and instead of showing the outline of a crown, they instead showed a star on one side, and a crescent moon on the other side.
"He looks a lot like you," Mickey murmured.
"Actually, the reverse is true. Same goes for Esan. There was something else I had to tell you too..." he trailed off, looking skyward for a moment, then with a snap of his fingers, he half-shouted, "That's it! You're also looking for the One who is Not, aren't you?"
"How did you know?"
"I wrote the inscription you got that from."
"Who does it mean?"
"A young girl you call Naminé. I think that's all. Now you'd better get going before the Darkside gets here. Leave that thing to me."
Sora nodded, murmured, "Thanks," in passing, then he and Mickey pushed onwards, headed for the fourth of the islands.
"Are you sure we should trust him?" Mickey asked him. "He did pretend to be Vanitas."
"I don't think he had a choice in that," Sora disagreed. "That was just an echo that overlaid him. I think he was just waiting for us. We've got a problem though, if he's right."
"Which problem's that?"
"Naminé is going to need all three Kingdom Keys, and I'm going to have to go after Seifer to get Kairi."
"Mind if I offer a solution?" Sand's voice asked, as he ported nearby falling into step beside them.
"In a moment. Update report?"
"Collective agents have been picked up in the worlds of all six of the other princesses of Heart and in Radiant Garden, unconfirmed sighting of Seifer in Atlantica but no sign of Kairi. I'm afraid we've heard nothing of the Cornerstone as yet, your Majesty."
"Good work all the same," Sora complimented him. "Now, your suggestion?"
"Borrow someone else's Keyblade, and have them borrow your Kingdom Key."
"Wouldn't work," Mickey shook his head. "The Kingdom Keys choose their wielders."
"Actually, that's not entirely true," Sora disagreed with him again. "Riku could wield my Kingdom Key, and he's borrowed it once or twice since then."
"Riku only has the Way to the Dawn Keyblade though, and that's more useful in his hands than yours."
"True. I think Ventus might be able to use it too though. His Heart did spend all that time with mine, remember."
"What about the keyhole that Esan told you about?" Mickey asked.
"Your key can lock keyholes too, can't it?"
"I guess I forgot for a moment," he murmured, faintly embarrassed. "Sand, do you think you could bring Ventus to us?"
"Sure thing," he answered. "He took his friends up to Ansem's Mansion to protect it in case the Collective tried anything."
"We should have thought of that," Sora shook his head ruefully. "Fetch him for me, Sand. Tell him what's going on as you go, have him get his Keyblade ready to exchange for mine, then get him back to the mansion as soon as the changeover is done." Sora glanced over at the next island, then added, "You've got about ten minutes before we hit the next echo. Get moving."

Ansem's Mansion
Basement

"Something's coming," Ienzo warned Ven. "Readings suggest it's a Heartless."
"Aqua? Terra?" he asked shortly, worried.
"They're swamped in them, but it looks like they're handling it. Is your Keyblade..."
Ven shook one hand, Wayward Wind appearing in it.
"Looks like it's alright for now. I'll be just outside," he added, then closed the door behind him.
Moments later, two Neoshadows stalked around the corner. They spotted him and charged, but they were little match for a few well placed strikes on each. Almost as soon as they disappeared, one wall pulsed and Sora's sometime messenger Sand stepped out.
"You've got your Keyblade out already," he nodded. "Good. Come with me, quick."
"What's happening?"
"You're going to be a bit more special than before," he replied with a faint grin. "Sora's going to lend you his Kingdom Key and borrow your Keyblade from you."
"Will that work?"
"Sora and Mickey think so. I'll explain the rest on the way back, you'll have to run with me for a bit and be ready to swap Keyblades on short notice."
"No time to waste then. Lets get moving."
Sand took his free hand, leading him down the corridor at a run into a pulse of light at the end. The view was purely white for a few moments, then cleared to a sandy bar, just beside Sora. Sand let go of his hand.
He handed Wayward Wind to Sora in one hand, taking his Kingdom Key in the other, then Sand grabbed his shoulder again and they were running back down the corridor, slowing.
"Quick," Ven noted, breathing slightly heavier. "You said there was more?"
Sand nodded, "Naminé is the One who is Not, and that means all three Kingdom Keys have to be present. Seifer's been revealed as a traitor and a Collective member, and kidnapped Kairi, so Sora's going to go after her while you, Mickey and whoever holds the third Key help Naminé."
"Makes sense. Might be a problem though."
"Oh?"
"My Keyblades have been a bit... unreliable. More me than them actually. Sometimes I can't wield any Keyblade at all."
"We'll have to hope it doesn't happen while Naminé needs you then. Can't stick around for much longer, I'm on orders to get back to Sora right away."
Ven nodded, watching him leave, then experimentally swung the Kingdom Key a few times. It somehow even felt powerful in his hand.
Two more Neoshadows rounded the corner. This time, a single stroke on each was enough to fell them.
"Formidable," he murmured.
The door behind him hissed open, and Ienzo glanced out.
"Did something just happen to you?"
"Sora and I exchanged Keyblades, why?"
He glanced down, noticing the Kingdom Key in hand.
"Interesting. What for?"
"Naminé really is the one we need to take on the Collective, except he's going after Kairi because Seifer kidnapped her, so I'm taking his place here with the Key. Do you want to tell me why you're so curious?"
"I've been monitoring you since you mentioned your unreliable Keyblade. You were right, it was your incomplete Heart that was making it unresponsive at times, but since a few moments ago your Heart just became mysteriously complete."
"Sora's Kingdom Key. It must have finished the job for me."
"Seems like it. Stay here though, if nothing else so I can keep a watch on you."
But Ven shook his head, "The Kingdom Keys are more effective against the Heartless. I've got to go help my friends."
"At least be careful."
"Naturally," he grinned, then left, headed up to relieve the load on Terra and Aqua.