i dont think Nero was born with the arm...

 in the Give me more power! cutscene read each part.

while Nero is pinned against the wall and the armors are about to kill him, the screen glows black, and you hear Nero's voice...

(Nero)"Kyrie, run!"
(Kyrie)"Nero"
(Nero)"Kyrie! Kyrie!!"

scene goes to Nero DTing and Yamato coming to his arm...

(Agnus)"How? Not even I could succeed in restoring it...!"

(Nero)"From that day forth... my arm changed...And a voice echoed...'Power...Give me more power!'"

(Agnus)"What?"

(Nero)"And if I become a demon, so be it. I will endure the exile...Anything to protect her."

 wanna throw this out there, from the beginning scenes Nero has the DB arm, but does anyone know something that might have happened a long time ago? he says, "From that day forth... my arm changed..." then, "And if I become a demon, so be it. I will endure the exile...Anything to protect her." did he at one point save Kyrie? but to save her, from whatever, somehow he obtained the DB and some kind of Sparda power? He isn't named a "blood relative" by anyone except "His Holiness" but even then i dont think he is blood of Sparda (Dante & Vergil). there was a post earlier (cant remember here or another forum) where someone stated that at the end of 3, Vergil was in the demon world bout to fight. then the story goes to dmc1 to Nelo Angelo (possessed by Mundus), then the story of 4, where Vergil seems to be gone 100%, but somehow when Nero fought to save Kyrie, b4 the actual game, he might have somehow gotten Vergil's power.

any comments or thoughts?

Dante & Vergil:

We can assume that something bad happened to Nero's arm, trying to protect Kyrie, when Nero blacks out right before Yamato reforms and flies into Nero's hand. And with Agnus's experiments, he was trying to extract demonic essence, and to make the knights, he used "a powerful devil's arm" which is clearly Vergil's Yamato. What I think is the Order "fixed" Nero's arm by extracting Yamato's essence, to test whether or not the experiments would be a success, thus why Yamato flew into his hand, and it might explain the "blood of Sparda" bit.

This is a theory I posted in another thread here. I have read all of your posts and I still have an inkling that this may be what happened, but it's only speculation. Remember that much. Just let me know what you think.

Take The Quiz Yourself!

he had his arm in the cast because he didn't want everyone to know about it, most importantly, Kyrie. Notice when she found out about the arm "being part demon" Nero kicked himself over it and said to Kyrie that he was "sorry"

So obviously Nero has had this,  you just can't "GET" a demonic power out of no-where...seriously. You either have it, or not. 

that cancels that out, moving on to other theories. 

Hey everyone come up with new idea's i'm getting tired of everyone saying Vergil possessed Nero's arm and that Nero's arm and D.T. looks the same they look nothing alike Berial's arm looks more like Nero's then Vergil's and Nero D.T. if it does look similar "which it doesn't really" is because the devil bringer absorbs powers and other demon objects. So hints that could be how Nero has powers of Sparda because Sparda made the sword and used it to close the portal between the human world and demon world. And for those who think Nero is Dante and Vergil little bro then answer me this why does Dante and Vergil heal almost instantly when they get wounded and Nero didn't even start healing from his injuries till he obtained the Yamato and Devil Triggered for the first time and after all that he still can't heal as quick unless he in devil trigger form.

Kayle Coyote:

Let's think about that one for a second, shall we?

#1) There ain't any evidence of a Sparda Family Tree past Sparda himself, Eva, Dante and Vergil.

#2) Nero can't be related to Sparda, simply cause the storyline wouldn't work.

#3) I've said this before, but there are several things that point toward Vergil's Devil Form takin' possession of Nero's arm.

        -: The markin's on the Devil Bringer are the same as Vergil has on his Devil Form.

       -: The Devil Bringer glows blue, as Vergil's Devil Form does.

      -: Nero's had a voice sayin' "power, give me more power" ever since he's gotten the Devil Arm.

         -: Nero's Devil Trigger has an underlyin' voice/growl, which sounds like Vergil's Devil Form.

         -: The Yamato Blade reforged itself in Nero's presence after he kicked Agnus' ass.

         -: The Yamato Blade also flew towards Nero's Devil Bringer, which is his right arm. Vergil is RIGHT HANDED.

#4) Although Dante has said he needs to keep Yamato "in the family", he also lets Nero keep it cause he thinks Nero's capable of havin' it to himself.
 

That really is enough to prove that nero has the blood of sparda, maybe it means that nero is vergils son

Firstly, Nero is one of the more humble characters in the series. The "Give me power" voice was obviously an external factor, and of all the devils in the series, it seems to fit Vergil the best.

Secondly, for those of you who said that Nero may be Sparda's son as well, the major flaw I find in that theory is that Nero is just too young. His age in DMC4 is the age that Dante and Vergil were at in DMC3. Also, I think Dante saw the possibilty of Nero having Vergil's essence. Look at the scene carefully when Nero enters the forest and Dante lets himself fall over the cliff. That fall, the entire motion, looks isentical to Vergil's fall at the end of DMC3. MAybe he was trying to see if it would awaken any memories in Nero?

Thirdly, I dont know if im imagining this, but if you look at the ghost of the Devil Bringer during a snatch or buster, it's gloved. Like Vergil's glove.

Another thing, I think that there is significance in Yamato. I know the DB cans absorb demonic artefacts, but thats all it can do. It didnt just absorb Yamato, it attracted it and fixed.  It hasnt attracted any other artefact so strongly.

Although to disprove Nero is Vergil, recall that Vergil absolutely hates guns. Doesnt view them as the weapons of a true warrior. But Nero has the Blue Rose. Maybe they just had to include a gun to keep it feeling like DMC.

Also, check out what happens were Nero uses the Blue Rose in Devil form. You get an attack known as summoned swords, were swords are formed from pure demonic energy and hurled at your opponent. Vergil is the only devil who had this attack, and he used it in Devil May Cry 3 special edition.

And I must say, that devil looks a helluva lot like Nelo Angelo. Too much so for coincidence