Coco Adel
Devil Arm - Pandora
Info - When her old weapon was damaged, Pandora called out to Coco to other her a chance to help her friends. When she accepted the other Pandora began to bond with her Aura, infusing it with demonic energy, booting both her power and semblance. Now her semblance doesn't just boost the effects of dust bust also gives dust like effects to her weapons now e.g fire, ice and lightning. Thanks to the demonic upgrade, Coco can both summon and dismiss Pandora and any point she desires. Now if only she could turn it into a handbag.

Mercury Black
Devil Arm - Gilgamesh
Info - It is unknown as to how Mercury managed to gain Gilgamesh but one thing is certain, the Devil Arm has bonded with Mercury's Aura increasing his power and somehow restored his legs although it's unknown if his semblance was restored as well.

Penny Poliander
Devil Arm - Lucifer
Info - after being damaged and her weapon destroyed in battle, Penny got separated from her friends only to hear something calling out to her. Following the voice lead her to a cave with a strange looking backpack and when she reach out to touch it, the pack sent out a wave of energy through her arm. "I am Lucifer and I can fix you." It said as it sent more demonic power coursing through Penny's body. Accepting the offer, Penny picked up Lucifer and put it on her back and in doing so allowed Lucifer to follow through with its deal, sending demonic waves of energy into Penny's Aura causing a strange feeling to fall over the girl as her body began to change. Using her soul and some scraps of D.N.A left behind in her construction Lucifer was able to artificially turn Penny into a Human-Demon Hybrid. Lucifer acts similarly to her old weapon and like Coco, Penny can also call on Lucifer whenever it's needed.

Yang Xio-Long
Devil Arm - Balrog and Cavalier
Info - 
Balrog- With Ember Celica destroyed, Yang was forced out of the fight until she could repair it but a mysterious voice called out to her and guides her to an unknown section in the Forrest of Death. In a cave she comes across the Devil Arm known as Balrog, the weapon beckoned her to put it on and as soon as she touched it a voice called out to her, "Should you choose to awaken me, you shall gain great power but you would have to learn to fight in a manner befitting of me." It boomed out. Thinking for a moment Yang accepted the deal and picked up the gauntlets. Suddenly a ring if fire appeared around her as demonic power started pulsing into her Aura, the weapon began disappear only to reappear on her body as her semblance activated. Once the process was finished the fire disappeared and Yang fell to her knees, "It is complete, you know wield my power but you still have much to learn before you can use my full strength." The weapon said as Yang slowly rose to her feet. After that, Balrog had started teaching Yang about Capoera and thanks to the demonic upgrade Yang received she had begun to learn at a rapid pace eventually mastering the style by the end of the week. The weapon, similar to Coco and Penny's weapons, can also be summoned at any point Yang desires.
Cavalier - While driving on the way to Atlas, Yang was attacked by an armoured demon knocking her off her bike. During the battle Yang's bike gets destroyed enraging her to the point that she unleashes her semblance and tears the demon to shreds sending parts of its demonic armour all around the place with some of it fusing to her bike reforging it. Now she has a bike that can turn into twin saw blades at any point with the additional bonus of it turning into the fastest bike that Remnant has ever seen. If it weren't for the demonic upgrade she received from Balrog, Yang would never have been able to wield the full power of Cavalier. Shame all it took to get this sweet ride was to completly destroy Bumblebee but hey at least it's still yellow and looks way cooler than it did before.

Pyrrha Nikos
Devil Arm - Draupner
Info - In a fight against a demon, Miló (Pyrrha's spear) is damaged badly. In a final effort to beat the demon, Pyrrha drives Miló into the demons chest wounding or badly. Accepting that Pyrrha was stronger the demon offered her its power before dissolving into a bright light and merging with her weapon, once the light faded Miló was no more in its place was a single ring. Puttin the ring on her finger, a vice called out to her in a whispered tone, "I am Draupner, the spear of divine wind." Upon hearing the voice a spear formed in Pyrrha's hand and with it came came knowledge on what the weapon could do. A powerful weapon with control over the destructive force of winds, a deadly combo indeed. Pyrrha is capable of summoning an infinite amount of spears that she can detonate at any point. Her shield was also upgraded when she received Draupner, no longer was it just a simple shield but it could turn into a watch that was capable of changed forms to meet the whims if the user. The shield was rechristened the Guardian Shield.

Sun Wu Kong
Devil Arm - King Cerberus
Info - Sun was simply wandering around when he came into contact with a pack of Grimm that unfortunately knocked him down into a hole. This hole contained King Cerberus and he wasn't to pleased at being bothered again especially since he was killed before and only managed to regain his form three years ago. He attacked Sun within seconds of seeing him but being the crafty Monkey Faunus he was, was able to outsmart the best and turn its power against it self before he delivered a powerful blow to its head killing King Cerberus while also destroying his weapon, Ruyi Bang and Jingu Bang but the demonic energy of Cerberus latched onto their remains and reforged the weapon and fused with Sun's Aura pumping it full of Demonic energy, merging with him to help him fully utilize the weapons power.

Cinder Falls
Devil Arm - Devil Sword Dante/Bahamut
Info - It's unknown as to how she got Dante's sword but with it both her Aura, Semblance and her half of the magical power of the maiden was boosted. Not much else is know as of the moment. Although at some point she did decide to rename the blade Bahamut but it is unknown if it was her idea or if Dante's soul had someone managed to whisper the idea into her mind.

Adam Tauros
Devil Arm - Murasama
Info - After slaying a large demon, Adam buried his blade in its corpse allowing the blade to a absorb the demonic remnants of the demon. Now transformed the blade lost its gun ability in favour of stronger attack power capable of burning so hot it melts through even the strongest of materials but Adams Aura and Semblance was also boosted as well.

Ruby Rose
Devil Arm - Nevan
Info - While not one for musical instruments, Ruby makes an exception for this Devil Arm. While on her way home for the holiday, Ruby was attacked by Nevan an electrical bat controling demon succubus. The battle wasn't anything of note as it didn't last long but Oum damn was the reward good. Upon defeating Nevan, the succubus lent Ruby her power upgrading the girls Semblance so that now not only when she activates it does she turn into rose petals but she also generates lightning that can stun some opponents (side note: this doesn't work on people with semblances similar to Nora). Now she's faster and stronger than ever before look out world Ruby Lightning Rose is here to help! Now, if only the Guitar Scythe was red though maybe she could repaint it.
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If there are any characters you want to see with Devils arms than just ask. But if you do than please tell me what Devil Arm you want them to have and if it's from a different franchise than tell me where it's from. If the weapon is an original idea tell me it's powers, what's it's name and who you want it to go to and I'll do it if I like it.