Rated T to M.
Esper Dragon Part 1: Bahamut's Arrival in DxD. At the end of Final Fantasy 6 Bahamut and the other Espers had faded away after the death of Kefka Pelatzo when he became the God of Magic, but what if Bahamut had survived? Bahamut after nearly fading from existence survived by a miracle and decided to travel the cosmos to enjoy his freedom. Bahamut after traveling throughout the cosmos eventually arrived to DXD before the Three Factions had started their war and just after the creation of the DxD world and when Bahamut arrived with his immense powers he created a paradise world that is twice the size of Planet Jupiter and made a separate plane of existence to hide and place that world where dragons are free to roam without fear of being hunted or having to tie themselves to any faction to survive and prosper as a species. Bahamut also returned temporarily to all the worlds of Final Fantasy starting with the world of Final Fantasy 6 and rallied the dragons and other sentient mystical beings and other sentient races from those worlds to live in the world he is making and also many dragons from the DxD world eventually come along and follow him along with the Fairies and other supernatural creatures where they do not need to tie themselves down to any supernatural faction and eventually building a world that blends magic with science. Also when Bahamut was creating this world he became the Father of the Neo Esper race and the progenitor of a whole new race of dragons. Now when the war of the Three Factions starts with the Original Devils slaying the Messiah(see A Demon Among Devil's by The Crimson Lord)Bahamut steps in when the conflict threatens to spill into the world of Terra Draconis which and fights both Albion and Y Ddraig Goch and holds his own against both of them at the same time without effort due to them making the situation worse for Dragon Kind in DxD and Terra Draconis as well as other supernatural creatures moving to Terra Draconis and calling it home. When the Three Factions team up to seal away Albion, Ddraig, and Bahamut, Bahamut has to be the only one of those dragons to immediately break free from his would be prison and annihilates all the Maou in retaliation and Bahamut strikes a deadly blow on God Himself that eventually kills Him and destroys DxD Bahamut. Also he wipes out the entire Olympian pantheon when they try to subjugate him and control his world.
Rated T to M. Esper Dragon Part 2: The Might of Bahamut. Bahamut when traveling throughout the Cosmos eventually comes to each of the Final Fantasy Worlds at the end and acquires all the powers, and assimilate the essences of all of Bahamut's alternates on all the Final Fantasy Worlds at the end of their games when he is traveling the cosmos. This possible with the Bahamut alternates souls merging in perfect harmony with his own, gaining their knowledge, techniques, special abilities, and powers added to his own, can transform into their various different forms, take a human form, and Bahamut can do everything he could do in all the games that he appears in including the Non Final Fantasy games. Bahamut after some intense training learns how to go into a Outrage Mode and gain the DxD Dragons truly endless life span and Bahamut gets even stronger as he get's older. Also Bahamut in terms of power in the DxD hierarchy is above Albion and Ddraig and is the strongest dragon in the world and is immune to dragon slaying weapons and magic and to everything else.
Rated T to M. Esper Dragon Part 3: Bahamut's Personality. Bahamut in terms of personality has to be strong willed, fierce, proud, but caring for his people along with being a good leader who puts himself in the fray when things get ugly for the sake of others and Bahamut is also a good father in that he cares for his son's well being and will do all that he can for his son and other children. Bahamut also has to have the sense of values of a mature man with immeasurable wisdom and when situations aren't serious and when he is not working on managing an empire that spans an entire planet Bahamut is rather playful and is a trickster pulling harmless pranks. But if people prepare for war against his or attack him and his family he shows no mercy and becomes a unstoppable force of nature against his enemy.
Rated T to M. Esper Dragon Part 4: The Son of Bahamut, Seiryuu, his personality, origins, powers, and role in the story. Seiryuu has to have the DxD special of a DxD Magician, Senjutsu, and Nordic Runes, Gandr, and Seidr along with Seiryuu learning all the full powers of the Final Fantasy's Black, White, Red, and Blue Mages from all the Final Fantasy games along with all the powers that Bahamut has. Now just to not make Seiryuu a carbon copy of his dad Seiryuu has to obtain these abilities such as having immense durability, being able to go into an Outrage Mode, learning Elemental magic, Alchemy, Nordic Runes, Shamanism; such as astral projection, conversing with Animal, Plant, Ghosts of the departed, and Nature spirits and evoking those spirits for help in battle and healing his allies and for knowledge, and finally Senjutsu. It would be awesome for Seiryuu to learn Senjutsu as DxD Dragons are masses of pure power given form from the creation of the world itself. Senjutsu involves using one's life force in harmony with the energy of Nature creating a current of power. While it does not stack up to Demonic Magic or Holy Light in sheer destructive power, the current is constant, add Senjutsu with all the other powers that Seiryuu has and we have a beast who can hold out till the end of the fight. Seiryuu at the end of the story has to be able to keep a level head throughout dangerous situations and be very courageous and intelligent along with being loyal, honest, free spirited, a tad mischievous and a closet pervert. But in the beginning of the story when he stumbles upon his powers and heritage at an early age he has to wonder where he belongs in the world as he is a half Esper Dragon, half Human Demigod. Seiryuu in the beginning is an emo due to thinking that he is monster and that his parents did not want him but he can be a bit hotheaded if you call him a monster, a freak, or mess with his friends and family. Seiryuu also has to be more solitary than Issei and is a tad antisocial in the beginning of the story due him thinking that freaks do not deserve friends and him thinking that they will shun him if people realize what he is. Seiryuu's role in the story he has to be one of the main characters and go through the DxD storyline along with his brother but not become a Devil. And for Seiryuu's origin when Bahamut after one of his business ventures in the Supernatural Worlds, took a vacation in the human world and fell in love with and eventually married and later knocked up a human witch after a beautiful and loving relationship that blossomed for years. She has to be in terms of personality and appearance a mixture of Risty from Queen's Blade and Bayonetta, from Bayonetta in looks and personality with a bit of Kushina from Naruto thrown in as well in terms of personality. However she died in childbirth making sure Seiryuu was born into the world. Bahamut worrying for his son's safety due to his status of being the son and heir of The True Dragon King, The Black Dragon of War, The Terror of the Skies, and The Black Dragon of Power and Calamity, The Black Dragon Abomination, The Black Dragon God-King(Final Fantasy Bahamut's Titles)decided to leave him as a baby with the Hyoudou family who then adopted him and raised him as their own along side Issei.
Rated T to M. Esper Dragon Part 5: Seiryuu and Issei's Harems.
Seiryuu's Harem: 1. Kuroka. 2. Rosswiess Number 3. Ravel Phenex Number 4. Akeno. Number 5 Kalawarner. Number 6. Xenovia. Number 7. DxD Tiamat. Number 8. Valerie Tepes
Issei's harem: 1. Rias Gremory 2. Koneko 3. Asia. 4. Irina
Rated T to M. Esper Dragon Part 6: Seiryuu's enemies. As the son of Bahamut Seiryuu has to have enemies from both angelic camps and the upper echelons of the church as Bahamut was the dragon who caused God's Death and when the Church finds out that Bahamut has a kid they end up vilifying and demonizing Seiryuu so they can have an excuse to have his head on a silver plate. So Seiryuu scoring popularity points with the Church and The Angels of Heaven is not going to happen. Also they would see him and Bahamut as abominations because they somehow found out about his father from being beyond the Dimensional Gap and how the Espers were created to be living weapons for the Warring Triad and Seiryuu's own with both being feared by the Angels both Pure and Fallen for Seiryuu's own potential due to his heritage with some of them wanting to kill him for what his old man did and hating his father but Bahamut is so strong that no one can really harm him so they attack those weaker than Bahamut. The Hero Brigade should want Seiryuu's head on a plate so they can have the honor of slaying a powerful dragon and the Old Satan Faction wants to control him as a living weapon against their enemies like the original Espers of long ago. Also some of the Angels when the Peace Summit goes down successfully want Seiryuu's head on a pike and when they cannot get some revenge they become Rogue Angel's(not Fallen Angels)who join the Chaos Brigade to remake the world in their own image and what they thought God Himself would want the world to be by killing all those from different religions and those with Forbidden/Heretical Sacred Gears.
Rated T to M. Esper Dragon Part 7: Seiryuu's weakness. As Seiryuu is a Half Human Half Esper Dragon Demigod has to be weak to Dragon Slaying Weapons but he does have the capacity to overcome that weakness to the point it becomes little more deadly than a bug bite at the end of the story. However until then when Seiryuu is struck with a Dragon Slaying Weapons or Dragon Slaying magic if it is not a death blow or he is not hit on a fatal spot on a normal person he can power through it midway in the story but still cause him much pain. But until Seiryuu get's stronger when training with all the Dragon Kings including bonding with his dad it is his one weakness.