Mars: once lush and terraformed, now fading back into a red, dusty desert. Many of mankind's holdings there had been destroyed at the collapse of the Golden Age. Huge complexes that had once flourished in commerce and industry had faded into the red sands, leaving only irregular dunes behind, some of them miles high.

But there were ways inside, for the determined.

A single ghost wandered the ruins beneath the Martian surface. She was a little robot shaped like a star with a single blue eye in the center, and she'd been there a long time. But her patience was inexhaustible. Somewhere in these ruined rooms and collapsed passages lay the spark of her Guardian, an immortal warrior.

She had hunted him for nearly a year, now, and had mapped huge portions of the complex. It had once been part of a city with thousands of inhabitants. But they had fled the Darkness to die in the asteroid belt, leaving their grand city to rot and rust.

Why her Guardian had died here, the ghost had no idea. Had he failed to evacuate? Had he come here later to search for treasure or supplies, and met with some accident? The ghost had plenty of time to dream of his past, what he might be like. Or she - it was hard to tell from the distant song of a spark.

But finally, late one cold Martian night, the ghost wound her way into a collapsed building where her Guardian's remains lay.

He had been a male Exo, she saw at once - a robot body with a human mind uploaded to it. Despite lying some distance from the fallen roof and walls, half his metal body had been torn apart and scattered across the room. His remaining hand still gripped a metal pipe - possibly he had used it as a weapon. The pipe and the hand that held it had fused together with rust.

The ghost flew to and fro over her Guardian, kindling his spark with her own, studying the physical damage. Then she opened her shell and bonded her spark to his, making herself part of his very soul, giving herself the power to heal and resurrect him. She would be his closest friend in time to come.

But he couldn't return to life with his body in pieces. The ghost poured Light energy into the Exo body, rebuilding components from local quanta. She reassembled his chest, pelvis, right leg, and right arm, along with all synthetic organs. The hand that gripped the pipe became new again, freed from the imprisoning rust.

Then she clothed him in a basic enviro-suit with a light helmet. The Martian atmosphere, terraformed at one point, was thin, but breathable.

Once she was certain his body was intact, she recalled his spark, and with it, consciousness.

The Exo's eyes slowly lit up in a blue-purple color. His face was green, which contrasted oddly. He stared up at the ghost.

"Hello," she said softly. "I'm your ghost, and you're my Guardian now. Can you move?"

He shifted his arms and legs, then sat up, rubbing his head. "What happened?" he said in a deep, gruff voice.

"I don't know," the ghost said. "I found you this way. Your body was half-gone."

"Half-gone," he murmured, looking at his hands. "Ghosty, come closer. I need more light."

She obligingly flicked on her headlight setting, illuminating him in harsh white.

"Ugh, not that much," he muttered. But he held his arms up to the light, turning them this way and that. Then he gripped his own left arm, peering closely at it.

The ghost looked, too.

Scratched into the metal that formed the inside of his wrist was the word Aestivalis.

"Aestivalis," the Exo repeated. "It's important. But ... I don't remember. Why don't I remember?" he demanded.

His ghost replied, "I had to rebuild large portions of your brain. Most Guardians don't remember anything of their pasts. Do you think ... maybe ... that's your name?"

The Exo sat there, shoulders hunched.

"Valis," he finally said. "I don't dare use the full word. I wish I could remember." He scrambled to his feet and gazed at the collapsed room around them. "Where are we?"

"Some kind of industrial complex, I assume," said his ghost. "On Mars."

"Mars!" Valis smacked himself in the forehead. "The Exo people. C-Clover-Cloven-"

"Clovis Bray?" said his ghost.

"Clovis Bray!" the Exo exclaimed. He ran straight at the nearest wall of collapsed debris and began digging at the sandy floor.

The ghost watched him uneasily. "If you get buried, I can't dig you out."

Valis ignored her in his feverish obsession. "It was five feet from me. It couldn't have gone far. Hey, robot girl, how long was I out?"

"Robot girl?" the ghost replied, indignation rising in her. "I'm your ghost."

"You sound like a robot girl," Valis said, still digging. "A cute one, you know? Anyway, how long have I been down here?"

"A very long time, judging by the amount of corrosion," the ghost said, suddenly unsure about this Guardian's stability. "Several centuries?"

"Centuries?" Valis exclaimed, sitting back on his heels to gape at her. "You're kidding, right? I thought you were some rescue drone they sent into disaster areas. How'd you reboot me after so much time?"

"I need to explain Guardians and ghosts," she said. "You've been through a resurrection, not a reboot. In fact, your reboot count stands at two."

Valis waved a hand. "Later, later." He kept digging, and his hands clunked against something glass. He pried it out of the sand and held it up.

It was a diamond-shaped pylon the size of his head. A series of lines and lights ran along its equator, long gone dark.

The ghost recognized it at once. "That's ... one of Rasputin's sleeper nodes. How did you know it was there?"

Valis set the pylon in his lap and stared at it. "Clovis Bray. Rasputin. Who's Rasputin? I should know this."

"The Warmind," the ghost said. "One of the great Golden Age AIs designed to protect Earth from extraterrestrial threats."

Valis ran his metal fingers over the node's surface. "I've lost so much," he murmured. "I used to know everything. I can't even remember why I needed this." He held it up. "Aestivalis."

To their surprise, the sleeper mode activated, green lights spreading across its surface. It floated into the air, opening and unfolding, playing an eerie music as it went.

A tiny black rectangle fell out of the node and landed in the sand. Valis snatched it up and examined it. "This. This was what I was looking for."

The ghost gazed uncertainly at the sleeper node, now awake and active. "I hope you haven't triggered something bad."

Valis stood up, gripping the black object. He watched the node spin slowly in place and play its music. "I'll bet I did." He glanced around the ruins. "How do we get out of here?"

"Follow me," said the ghost. "It's a bit of a walk, but we can find you some gear along the way."

Valis followed her up sagging staircases and down winding passages. He seemed to come out of whatever fugue state he'd awakened in. "This place is wrecked. What happened to it? Bombs?"

"Centuries' worth of decay," said his ghost. "You've been down here a long time. I've hunted through this place for you for eleven months, eight days."

"Wait," said Valis, halting. "Why?"

The ghost backtracked and studied him. "Because you're my Guardian."

"Yeah, you said that already. What does that mean? Am I some kind of military target?"

"No," laughed the ghost. "Let me tell you about the Traveler."

It was a long story, and they walked as she talked. Valis listened in growing disbelief. Alien invasions, Earth and its holdings depopulated, only one city left on the whole planet - in the whole solar system, even. And the Traveler, a paracausal entity that resembled a moon, sitting in the sky like a giant battery, pumping out space magic, otherwise called Light, to empower its resurrected army of Guardians.

"So, I was dead," he said flatly.

"Yes," said his ghost. "Dead for many years."

Valis looked at the black object in his hand. "I wonder if this is any use, then. Or if Aestivalis is important anymore."

"What is that thing, anyway?" his ghost asked.

He held it up. "Thumb drive. Memory storage. But there might not be any devices left that can read it."

The ghost flew up and traced it with a beam of blue light, scanning. "I can read it. It has a lot of data here, all about ..." She fell silent and turned off her beam.

"About what?" Valis said eagerly.

His ghost looked at him, flying right up to his face and scrutinizing his eyes. "What are you?" she whispered. "These are your memories."

"Good," he said. "What are they? Since obviously I don't remember them."

The ghost spun her shell one way, then the other, agitated. "We should wait until we're back at the Tower."

"Why?" he said. "Fill me in now so I don't go making a fool of myself, talking about what I do remember."

The ghost flew in circles. "I ... you ... this is data stolen from Clovis Bray, data you were sent to destroy. But you thought it was important enough to copy, first. Clovis Bray is still around, Valis. Ana Bray is running it. Rasputin is still operating. This data could endanger everything. Including us."

Valis watched her fly. "All right. I won't ask for more details. All but one thing. Is Aestivalis tied to that?"

The ghost nodded emphatically.

"Then I won't spread the word around," Valis said. "I'll have to figure out how best to proceed."

They continued traveling, winding their way across the ruined complex toward the exposed area where the ghost had entered. The Exo moved lightly, acrobatically, swinging around poles and crawling under obstacles without a thought. The ghost watched him and pondered what fighting discipline he might take.

"So, you're a ghost," he said suddenly. "This little piece of magic sent to make a Guardian and keep them alive forever."

"Um, yes?" the ghost said. "You could put it that way."

"So you're not really a robot. Or even a girl."

Nonplussed, the ghost stared at him. "The Traveler created me with feminine characteristics."

"Right," said Valis. "But you're not a robot, either."

"I have mechanical parts powered by the spark in my core," said the ghost, wondering if she ought to be seriously offended. "Similar to the way you operate, as an Exo."

"Huh," Valis said. "So you are a robot girl. For all intents and purposes." He poked her shell. "And you had to go look like this."

"What did you expect?" the ghost said acidly. "A cute human in a pink skirt?"

"Not saying I'd object," Valis replied. "What's your name?"

The ghost backed away from him in a huff. "I'd hoped my Guardian would name me, but I'm beginning to think that's a bad idea."

His purple eyes blinked off and on. "Wait. I think I'm expressing this really badly."

The ghost turned her back on him.

They stood there in silence for a moment.

Valis said, "I'm sorry. I didn't mean to hurt your feelings."

The ghost whirled to face him, her eye-light arranged in an angry downward V. "You're way past that, now. I'm regretting ever bonding our sparks, because now we're stuck with each other. I dreamed for so long about finding my Guardian, and so far, it's been terrible."

Valis studied the little robot and sighed. "We're both robots, and I still don't understand women."

"How about starting off by not being a creepy perv?" she snapped. "I'm your ghost. I'm part of your soul, now. Please don't make me regret it."

"I'm sorry," he said quietly. "I didn't mean it that way. I like your voice. I think it's adorable. I'm just having trouble parsing it with a tiny floating spotlight."

She hovered in silence, unsure how to take this.

"So," he ventured, "might I call you Sakura? It means cherry blossom."

The ghost turned the name around in her mind. It was a nice name with a pretty meaning. Not what she'd expected from Valis. Maybe she had misjudged him - they'd barely known each other a day.

"All right," she said, finally. "You may call me Sakura."

"Great," he said. "Let's get out of these ruins. I'm starting to feel claustrophobic."