The Welsh dragon (Welsh: Y Ddraig Goch, i.e. 'the red dragon') is one of the symbols of Wales. It appears on its Welsh flag and is probably a trace of the Roman colonization, whose army used dragons as banners.
Henry VII took the symbol from King Cadwaladr, placing it on a green and white background representing the House of Tudor, on his way to the Battle of Bosworth.
Although it has been linked to Wales and the red dragon for millennia, it was only in the 20th century that it became its official symbol.
According to tradition, the origin of the red dragon depicted on the flag of Wales, called Ddraig Goch or Welsh Dragon, comes from a very ancient conflict between two of these beasts, one white and one red.
The white dragon was said to be the incarnation of evil, but there was a problem, and that was that the constant clashes between these two dragons caused damage to humans, and it was believed that the simple sound they emitted when fighting was enough to leave those who heard it without offspring.
Llud, the then monarch of Great Britain, decided to find a solution to this great conflict, for which he asked his wise brother Llefelys for advice. Llefelys suggested digging a huge hole in the center of the kingdom and then filling it with mead, so that the dragons would get drunk and then be easier to kill. His plan worked halfway, as both beasts were trapped for centuries, but they were still alive.
A long time later a new king named Gwrtheyrn decided to erect a great castle over the dragons' prison, discovering both creatures still in captivity. Gwrtheyrn seeks advice from the illustrious Wizard Merlin, who advises the release of the beasts so that they can continue their battle.
Once freed by Merlin, the fight between the two creatures ended with the victory of the red dragon, so centuries later, King Wthyr Bendragon (or Uther Pendragon, father of the mythical Arthur of Camelot) decided to take the figure of the great red dragon as the emblem of his lineage and the country of Wales.
When there is a sunset in Snowdonia, the sky turns reddish, which brings to mind the great red dragon and the whole legendary era of Wales.
In the light novel, manga and anime Highschool DxD the protagonist Issei Hyoudou has the Welsh dragon "Ddraig" (or Red Emperor Dragon, Sekiryuutei) sealed in his SACRED gear "Boosted Gear", located on his left arm, while his antagonist Vali Lucifer has the white dragon, known as the English dragon "Albion" (or White Emperor Dragon, Hakuryuukou) sealed in his sacred gear "Divine Dividing" located on his back in the form of white wings.
Both dragons, as in the original legend were bitter enemies, so much so that they intervened in the war between the factions of God, the fallen angels and demons, therefore they decided to join forces to eliminate them, God managed to lock them in their corresponding sacred gears, which are given to humans giving them the potential to change the world.
Both bearers of the sacred gears of these dragons are destined to fight as did the various generations of bearers before them, and so will the bearers who succeed them.
In the Type-moon franchise, within the universe known as the Nasuverse, both dragons represent the struggle between the Saxons (White Dragon Albion) and Britannia (Red Dragon Y Ddraig Goch).
King Arthur (Artoria Pendragon) is the incarnation of the Red Dragon and the one who opposes Vortigern, the brother of King Uther Pendragon (Artoria's father) and the incarnation of the White Dragon.
In itself the White Dragon is the incarnation of Britain's will to reject the Age of Man (Age where magic and the supernatural are eclipsed by science and humanity) and turned Vortigern into his will and avatar against humanity.