DISCLAIMER: I don't own Prototype or Devil May Cry.
Dante vs Alex Mercer
Dante and Trish looked up from their pizza as Lady walked into the Devil May Cry agency. Trish turned to face her, a slice of pizza in her hand. "Well, well, well. To what do we owe this most distinctive pleasure? You need Dante and me to do your job for you again?" Ignoring the two of them, Dante started to reach for the last pizza slice, when he became aware of an unfamiliar, and most repugnant smell. He looked at Lady, appearing disinterested, but his mind was racing. By now, he knew Lady's scent better than he remembered his own, and there was not a trace of her usual scent on her.
Lady approached the desk, leaning on it next to Trish. Lady started to reach for the last pizza slice, which was directly between her and Dante, when suddenly her arm transformed into a tendril and lashed straight at Dante's heart. Having been expecting something like this, Dante grabbed the tendril before it reached him, pulling the demon that had disguised itself as Lady forward and gripping its throat. The demon's other hand grew large claws and swiped them at Dante, but Dante vanished in a red blur, and the claws simply shredded his empty chair. Dante rapidly picked up his sword and guns, and turned to face the demon.
Trish was already lying on the floor unconscious, having been struck in the head with the demon's shoulder when it had lunged across the desk at Dante. She would have easily withstood the hit normally, but she had been weakened by a fight with a large gang of unusually strong demons earlier that day. Lady's form faded, to be replaced by that of a tall, pale man in a grey hooded jacket with a black leather jacket over the top.
The man charged at Dante, swinging his clawed hands at the Devil hunter. Dante easily defended himself using his sword, deflecting all the demon's attacks with time to spare before driving his sword into the demon's chest. For several seconds, the two adversaries stood still, before the demon's claws were replaced with a massive blade, which he swung at Dante. Dante vanished, appearing behind the demon and firing a bullet imbued with Devil energy into the back of its head.
The demon spun around, the fist-sized hole in its face already closing, and struck at Dante again with the massive blade, which Dante blocked with his sword. The two huge blades let off sparks as they ground against each other with enough force to knock over a building. The ground the two monsters stood on started to split apart from the sheer force the two were exerting. Dante leaned slightly forward. "Tell me something, big daddy silverware. What kind of demon are you? I've never met a demon capable of assuming a specific person's shape that was also capable of turning itself into various pieces of oversized cutlery." The demon's other hand turned into a large, organic hammer, which it swung at Dante's face.
Dante leapt away, sliding across the floor. The demon advanced, its hammer turning into a bladed whip. "I'm not a demon, you fucking moron. I'm a virus. My name is Alex Mercer, and as for my shape shifting abilities, I can turn into anyone, as long as I consume them first." Dante narrowed his eyes, uncertain whether or not to believe what Mercer was telling him, and at the same time hoping he had misheard.
Smirking, Mercer went to the door, where Lady's motorcycle waited outside. Mercer reached into the weapons compartment and removed the two halves of a weapon Dante knew only too well: Lady's personally designed rocket launcher, the Kalina Ann. Smirking sadistically, Mercer held up the two pieces and threw them to Dante. Dante caught them, looking very shaken, his eyes filling with tears. Mercer took advantage of the momentary distraction, wrapping his tendril around Dante and pulling him over. Mercer stabbed Dante through the chest with his blade, before wrapping his tendrils around Dante and using both them and the blade to cut straight into his mid-section, the razor-sharp blades struggling to cut through the Devil hunter's amazingly sturdy body, and stopping dead at Dante's ribcage.
Mercer attempted to consume Dante's genetic code, but he couldn't, and he couldn't break his backbone to consume him, and despite Mercer's efforts, Dante's demonic blood was impossible to consume just as his bones were seemingly impossible to break. Just as Mercer realised this, there was a flash of red energy, and the wounds in Dante's body closed in an instant, his form turning into that of a red and black Devil rather than a man as they did so. The blast of energy staggered Mercer, and vapourised his tendrils and his blade, thereby removing them from the wounds. Mercer looked up to see a seven foot tall, black Devil with massive wings that looked both reptilian and insectoid. A powerful aura of red Devil energy crackled around the monster, and a murderous glare radiated in its glowing red eyes.
Mercer started to advance, but with an inhuman roar, Dante charged into him, grabbing Mercer with razor sharp claws and taking flight, shooting straight up into the air, his Devil energy burning a hole straight through the concrete ceiling as if it were paper. Mercer started desperately attacking Dante with every weapon he had as the red Devil carried him into the sky, moving faster than a bolt of lightning. Despite Mercer's best, most desperate efforts, his attacks were completely useless, as the field of Devil energy surrounding Dante's demonic form proved impenetrable, even for Mercer.
After only a few seconds, Mercer became aware of a change in gravity. He looked around him to discover that Dante had flown him into outer space. Due to the Devil energy that had been burning him all the way up, Mercer hadn't even noticed when they passed through the atmosphere. Mercer looked with terror into the glowing eyes of the Son of Sparda, which, by contrast, were filled with nothing but cold, grim determination.
The red Devil energy around Dante intensified, burning Mercer with more power than he had ever encountered. Mercer watched in horror as his body started to get vapourised by the energy. Mercer hardened his skin, transforming into his armoured state, so that he looked like a dark grey humanoid monster made of organic metal, but it was no use. Mercer's armoured state was solid enough to deflect anti-tank rounds, and his regeneration was sufficient to survive a nuclear blast, but Dante's Devil energy burned straight through the viral armour, which failed to even slow the energy down, and somehow the energy burns were not healing, almost as if Dante's power could bypass limitless regeneration. After several seconds, the energy had done its job, completely erasing Alex Mercer from existence, not leaving so much as a cloud of ash behind.
Trish came to just in time to see Dante, in his Ultimate Devil Trigger state, drop back in through the hole in the roof. "Did you children have fun?" She asked playfully. Dante didn't answer, reverting to his human form and heading upstairs to his bedroom. Trish followed him up, and found him lying on his bed with his back to her. Her highly sensitive demon ears heard faint sobs. "What is it?" She asked, serious now. Dante didn't answer, but the sobs stopped.
Trish stepped back out the door." If you like, I'll leave you alone. Whatever's wrong, you might find crying gets it out of your system." Trish prepared to close the door, but heard Dante say something she couldn't quite make out. She leaned closer to hear better, asking him what he had said.
Without turning around, Dante said, "I said I'm not crying. Devils never cry."
Reasons for this outcome:
1. Alex Mercer is strong enough to lift and effortlessly throw a tank, but Dante is strong enough to block a blow with force equivalent to a massive skyscraper falling at hypersonic speeds. Put simply, Dante is significantly stronger.
2. Alex Mercer has an incredible regeneration, but not only does it rely on him consuming organic matter, which would make it limited anyway, but Dante has been shown to effortlessly kill a demon whose regeneration was based on some form of magic, allowing the demon to reform from his own blood and use the blood as a weapon after being blown to pieces. This demon was also very powerful in other ways, such as strength and energy attacks, as well as what was either astral projection or a limited form of omnipresence, and creation of a portal that covered the entire planet. Put simply, that demon was far more powerful than Alex Mercer, and Dante called his abilities child's play right before effortlessly defeating him by using Devil Trigger, and generally regarding him as an annoyance rather than a threat.
3. The approach taken by Alex Mercer to enter an area is always to disguise himself, so it makes perfect sense he would consume the weakest person available, which is Lady. By Prototype 2, Mercer has also grown very sadistic, and it makes perfect sense he would taunt Dante with the fact he had killed Lady. Finally, Dante is immensely protective of those he cares about, and it makes perfect sense that upon hearing Mercer's story, Dante would be furious, and it should be noted that the first time he used the Majin form it was in anger at the fact Mundus had seemingly killed Trish. So Mercer's actions and Dante's reactions to them are quite realistic.