The general rule of thumb is that the character needs to have enough canon (through the main canon or additions and adaptations) to have a distinct personality and history. For example, the character of the original Ion in Tales of the Abyss is only mentioned a few times in the game - however, there is a manga chapter and additional information in the novels that provides enough information on his personality to make him distinct and appable.
If you'd like to ask about a specific character's status in regards to whether they can be apped, feel free to send the mods a more detailed question about them.
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The general rule of thumb is that the character needs to have enough canon (through the main canon or additions and adaptations) to have a distinct personality and history. For example, the character of the original Ion in Tales of the Abyss is only mentioned a few times in the game - however, there is a manga chapter and additional information in the novels that provides enough information on his personality to make him distinct and appable.
If you'd like to ask about a specific character's status in regards to whether they can be apped, feel free to send the mods a more detailed question about them.