“There are two dragons called the ‘Red Dragons’. One of them is the ancient dragon from Welsh residing in you. Welsh Dragon. The Sekiryuutei . Hakuryuukou  also comes from the same origin and the same myth. But there is one more ‘Red Dragon’. That is the Red-Dragon which is recorded in Revelation .”
“The True God-Emperor of Red-Dragon. Apocalypse Dragon, the Great-Red. It’s the great dragon which is called the ‘True-Dragon’. It chooses to live in the dimensional gap and swims there for eternity. Today, we came here to confirm ‘that’. The field of Rating Game is inside a section in the dimensional gap by putting on a barrier. This time, Ophis’s true aim was to confirm that. Shalba’s plan was something which wasn’t important to us.”
“But why is it flying in a place like this?”
“No idea. There are lots of opinions about it….. That is Ophis’s ‘aim’ and the ‘target’ I wish to defeat.”
That time, Vali showed the clearest eyes never seen before.
“The one I want to fight the most. The Apocalypse Dragon Great-Red who is called [DxD], Dragon of Dragon. –I want to become “The True God-Emperor of White-Dragon”. It won’t look good if the ‘white’ is a rank below the ‘Red’ when there is the ultimate-class for ‘Red’ right? That’s why I will become one. One day when I defeat Great-Red.”
DxD is initially thought to mean "Dragon of Dragons," referring to Great Red,
however it is revealed in the 16th Light Novel that DxD is actually the name of the anti-terrorism organisation that was formed to counter "Khaos Brigade," with the DxD meaning the different factions which form the group.