In many betrayed dragon fanfictions, there are other dragons too. Sometimes a dragon's mate can be a character. For example, in a story where a male dragon is betrayed, his female mate might be searching for him or trying to avenge him. There could also be dragon hatchlings as characters in cases where the betrayal affects the whole dragon family.
Humans in general are common characters. There might be a group of villagers who initially befriend the dragon but later betray it. Or a king or queen who orders the betrayal for political or economic reasons. There could also be a dragon whisperer type of character who tries to understand the dragon's side after the betrayal and help it.
Issei is often a central character in these fanfictions. Since he is the main protagonist in the original series, his relationships and potential betrayals are explored. For example, a situation where his friends or allies in the Occult Research Club turn against him for some reason, like jealousy over his growing power or a misunderstanding about his actions.
There are often brave knights. These are the ones who are usually at the forefront of the conquest, with their swords and shields ready to face the dragon.
The dragon itself is, of course, a common character. Whether it's in its original form at the start or in its reincarnated state.
One common theme could be trust broken. Since there is a betrayal involved, the shattering of trust between characters would be likely.
The dragon itself is, of course, a common character. There are often dragon keepers or handlers in these stories too. They take care of the dragon during the egg - laying process.
Spyro is obviously the main character. Then there could be characters like Sparx, his dragonfly friend, who might play a role in the harem dynamic. Also, there could be female dragon characters created specifically for the harem idea, each with their own unique personalities and abilities.
Sasuke could also be a character who is frequently betrayed in these fanfictions. Given his complex personality and his history of being both a loner and a seeker of power, he might be lured into dangerous alliances in the DxD world. And his partners in the DxD world could betray him when they realize they can't control him or when his power becomes a threat to them.
Harry Potter himself is very common. Writers often put him in situations involving dragon spirits to see how he would react and grow. Hermione Granger is also popular as she could use her intelligence to study the dragon spirit, perhaps trying to understand its magic or origin.
Naruto himself is often a common character. Since he is the main protagonist of the original series, it's natural for him to be involved in fanfiction about a new and powerful element like the Dragon Eye. Sasuke is also common, as his rivalry with Naruto can be extended into the story around the Dragon Eye. Kakashi might also appear frequently as a mentor figure who can guide the characters dealing with the new power.
Often, Harry Potter himself can be a character in these fanfictions. He might discover his hidden talent for taming dragons. Hermione could also be there, using her vast knowledge to help with the dragon - taming process. Ron could be the one who is initially scared but then warms up to the idea. And of course, Hagrid is likely to be involved as he has a great love for magical creatures, especially dragons.