The dragon itself is, of course, a common character. Whether it's in its original form at the start or in its reincarnated state.
Often, there are also antagonist characters. For instance, a dark sorcerer who wants to use the power of the sakura reincarnation for his own evil purposes. This creates conflict in the story as the protagonists must protect the sakura spirit from being exploited.
Goku is often a very popular character in these fanfictions. His iconic status in the Dragon Ball series makes him a natural choice for reincarnation stories. Writers love to explore different versions of him, like if he was reincarnated as a villain or with a different set of powers.
Naruto can also be paired with Sakura in these fanfictions. Since they are such close friends in the original, in a reincarnation scenario, their friendship could potentially turn into something more romantic. They could support each other in different ways than in their past lives.
Many fanfictions pair Ash, in his reincarnated state, with new characters as well. Sometimes there are powerful Pokémon trainers or even mysterious characters from different dimensions. These new pairings can introduce fresh storylines and dynamics, like the new character guiding Ash through his new, reincarnated journey or being in conflict with him over different goals.
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.
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.
The theme of identity is also prevalent. The reincarnated characters are in a new body but have old memories, so they struggle with who they are now. And, of course, there's the theme of adventure. They go on quests and battles just like in the original Dragon Ball Z story.
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.
The evil dragon itself is surely one of the main characters since it's in the title. There might also be a wizard or a knight who is either trying to stop the dragon or help it in its reincarnation journey.