If it's a creature or companion that is capable of having full comprehensive conversations with other people besides their owner - then yes they are considered appable characters!

To use pokemon for an example: Ash's pikachu is not appable since all he can say is "Pikachu". Meanwhile Team Rocket's Meowth would be considered an appable because he is fully capable of speaking and holding conversations with other people.

Since digimon are capable of human speech, they would be considered appable characters! Same with magical girl fairies, as those typically also speak.

Hope this helps!