I had an idea for My One Piece fanfic, but the series is too long. My OC name is Nico John. He is the identical twin of Nico Robin, meaning that he is the same age as her. John is also the son of Nico Olvia & Unamed Father, born into a family of archeologists. He & Robin are inseparable. They read together, study poneglyphs, even went to the Tree of Knowledge. But everything changes: They finally reunited with their mom, archeologists begin arrested, & the destruction of Ohara (similar backstory). John & Robin then fled to get a boat. They have managed to get off of the island, making the two of them survivors. Not long The World Governorment blames them for the genocide of Ohara, thus having a bountyon their head. When they found out they were dubbed "Demon Children," causing them to hate the government. John & Robin knew that the longer they stay together, the chances of them getting captured or killed. So, the first time since birth, they went their separate ways. While Robin has her life on the run, John been staying low. He has been stealing fruits & bread from stands, fishing in lake. He doesn't stay on an island for too long. Just a week. He fill a small box of fruits, vegetables, fish, as well as certain items to help him set sail. Not long, he arrived on Baltigo, home of the Revolutionary Army. Monkey D. Dragon then takes the boy, due to him being starved & dehydrated for three days, & brings him in headquarters. They help feed John & gave him water. Miss Belo Betty even bathe him, although he can bathe himself. Dragon knows that he might be one of the two children survivors. When he asked John what he wants to do, John wants to find a way to get stronger & clear their names. Dragon then decided to have him stay in the Revolutionary Army. To get stronger, John is training by Hack into Fish-man Karate, as well as joining the east army, led by Belo Betty. Even as a child, he was receiving some rude insults. However, he knows that she wanted him to make his choice. His love life gave him a choice: (Based off of Mirajane Strauss From Fairy Tail). She & her siblings, (Based off of Lisanna & Elfman also from Fairy Tail), was also in the Blue Army. John did not fall for Mirajane for her personality, but more than that. He doesn't know it. Dragon then take them to see Vegapunk, where they met Bonney. Dragon asked that if the Artificial Devil Fruits are ready. They are ready. Dragon gave them an assigned Artificial Devil Fruit. The siblings are a bit hesitant, but then saw John drink a weird green blood. While he drink it all, he hate the taste. John now has two Devit Fruits' abilities: Soul Soul Fruit & Revive Revive Fruit. The Strauss siblings received the Zoan-type Devil Fruit (Mirajane has Zoan: Satan, Lisanna has Zoan: Animal, & Elfman has Zoan: Beast). Vegapunk told Dragon that the Green Blood actually has a mixture of two Devil Fruits & wants him to give feedback to him. The four of them then train with new powers, with John focusing on his Devil Fruit. He then found out that his combination of the Devil Fruit has some of both: He can manipulate Souls, including his own. He can takes people soul & added to his own, gaining stronger & live longer, & thanks to Hack's Fish-man Karate, he found a way to manipulate water using his soul. He did lose his ability to swim. During his battle in the Revolutionary Army, he is getting stronger. He did went on his own to find more adventures & killing Marines, earning the new nickname, "Marine Killer." At Age 12, his adventures starts, taking out Marines(while taking their beri, which is the world's currency) & helping those in need, including destroying slave trading shops in Sabaody Archipelago. He then went back to Baltigo to reunited with Elfman, Lisanna, & Mirajane. He was shocked about the changes, especially Mirajane. She went from Gothic with temper to the nicest person. She asked John to be her boyfriend. He is happy, he said yes. They hang out for 6 months. For 6 months, they are promising each other to buy an island & getting married. However, that day will never come. Lisanna was about to take John to Fish-man Island, but she felt something bad. Mirajane is in trouble. Both John & Lisanna then rush to where Elfman took Mirajane: to Sabaody Archipelago. They split up hoping to find them. John & Lisanna then reunited, & found Elfman still breathing. John then rushed to see Mirajane Strauss, but not in her body, but her spirit. John asked Mirajane what happen. Elfman & Mirajane were ambush by Marines & their Admiral. They somehow found our relationship & demanded that she tell them. She refused. Then, Admiral Akainu then grabbed her & beat her, & demand to tell him. When she still refused, he did the unthinkable: use his Devil Fruit powers & melted her. This broke John as he then felt to his knees. The soul fragments are telling him that he is a pathetic man for abandoned the girl, that they could have their fun with her, that bad things happen when you're with the Demon Children. Mirajane told him to find another one, to help filled the void in his heart. The same way she did. As she was about to leave, John quickly takes her soul & absorb it. He locked her away, refusing to let her move on. Lisanna came back with Elfman's beast arm...what is left of it. John & Lisanna then left Sabaody Archipelago. John then sense Mirajane's memories & she is telling the truth. Now, bearing with grief & sadness, his quest for revenge is now set in stone. However, Monkey D Dragon sense his dark intention, but decided to see if a project could work. Dragon persuade John from going there now. Instead he should get even stronger. He wants to meet his son, Monkey D Luffy. He was a bit skeptical about this, but Dragon promised that he will be stronger than ever if he finds Luffy. Still skeptical, he then sighed & accept. He was getting everything he owns, get a boat for one & set sail. It took him years to find Luffy. He find him along with the pirate hunter, Zoro. Thus, his adventures as Straw Hat's vice captain begins.

That's a really detailed and compelling backstory for your One Piece fanfiction! Nico John sounds like a complex and driven character. You've laid out a great foundation with his unique Devil Fruit powers, his connection to Nico Robin, and his tragic love story with Mirajane.
Here's a breakdown of what you have and some ideas to consider as you continue developing your fanfic:
Strengths of Your Backstory
* Strong Emotional Core: The bond with Robin, the destruction of Ohara, and especially the devastating loss of Mirajane provide powerful emotional anchors for John's character development and motivations. His grief and desire for revenge are very clear.
* Unique Power Set: Combining the Soul-Soul Fruit and Revive-Revive Fruit is incredibly interesting and offers a lot of narrative possibilities, especially with the added ability to manipulate water through his soul. This gives him unique combat potential and ties into Hack's Fish-Man Karate training.
* Compelling Motivation: Clearing his and Robin's names, getting stronger, and avenging Mirajane are all strong, intertwined motivations that will drive his actions throughout the story.
* Ties to Canon: Integrating him into the Revolutionary Army, having him meet Dragon, Vegapunk, and eventually Luffy and Zoro, grounds your OC firmly within the One Piece world.
* "Marine Killer" Nickname: This is a cool and impactful nickname that immediately tells the reader a lot about his character and his methods.
Areas to Consider & Expand Upon
While your backstory is solid, here are some points and questions to think about that could add even more depth and impact:
* The "Unnamed Father": Is there any significance to his father being unnamed? Does John ever wonder about him? This could be a minor plot thread or simply a detail to consider.
* John's Perception of Robin's Life: You mention Robin has "her life on the run." Does John ever think about what she's been through? Does his experience with the Revolutionary Army and becoming strong make him feel more responsible for her, or perhaps resentful of her path, or simply want to protect her when they reunite?
* The Revolutionary Army Arc:
   * Training with Hack and Belo Betty: While you mention he's training and receiving insults from Betty, perhaps show more specific examples of this training. How does Hack adapt Fish-Man Karate for someone without fish-man physiology? How does Betty's tough love specifically shape him?
   * Growth and Recognition: How does John rise through the ranks, or at least gain recognition, within the Revolutionary Army before he leaves? Does he participate in any notable missions?
   * Dragon's Intentions with Luffy: Dragon's promise to make John stronger if he finds Luffy is intriguing. What is Dragon's specific goal here? Is he testing John? Does he have a grander plan involving both John and Luffy that he hasn't revealed? This could add another layer of intrigue to John's journey.
* The Artificial Devil Fruits and Green Blood:
   * Side Effects: Beyond losing the ability to swim, are there any other subtle or long-term side effects of the "green blood" and the artificial fruits, especially for John with his unique combination?
   * Vegapunk's Role: Does Vegapunk continue to monitor John's progress with his unique Devil Fruit combination, or is this a one-time interaction?
* Mirajane's Soul:
   * The Aftermath: What are the immediate psychological and emotional consequences of John absorbing Mirajane's soul? Does he hear her voice? Does her presence influence his decisions? This is a huge turning point and could lead to very interesting internal conflict.
   * The "Locked Away" Aspect: How does he "lock her away"? Is it a mental block, a conscious effort? What happens if he can't keep her locked away? This could be a powerful internal struggle for him, especially during emotional moments.
   * Impact on his Powers: Does absorbing her soul somehow enhance or alter his Soul-Soul Fruit abilities, perhaps giving him a deeper understanding of souls or even a connection to her "Satan" Zoan power?
* Elfman and Lisanna's Fate: You mention Lisanna returning with Elfman's beast arm. This is a brutal detail! What happens to Elfman? Is he alive but severely injured? How do Lisanna and Elfman cope with Mirajane's death and John's absorption of her soul? Their reactions and relationship with John afterward could be very impactful.
* John's Solo Adventures (Age 12+):
   * "Marine Killer" in Action: Show, don't just tell. Describe an instance or two where he earned that nickname. What kind of Marines does he target? How does his unique power set lend itself to this?
   * Helping Those in Need: How does he go about helping people? Is it subtle or more overt? How does this align with or contrast with his revenge quest?
* Reunion with Luffy and Zoro:
   * First Impressions: How does he meet them? What are their initial reactions to each other? How does his personality (grief-stricken, vengeful, yet perhaps still kind underneath) mesh with theirs?
   * Becoming Vice-Captain: What makes him suited for this role? Is it his power, his experience, his strategic mind from his archaeologist background, or his leadership qualities?
Continuing the Conversation
You've got a fantastic base here. What aspect of John's story are you most excited to explore first, now that he's joined the Straw Hats?