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Possession is an AU where Flowey can possess, or take over a single in-game character. Sometimes he can possess two, and sometimes more than two.
About the AU
Everything is also the same. The only difference is that Flowey can take over a character. The most common target that Flowey possess is Papyrus. The rare or least common ones are the other characters, like Undyne and Gaster. Sometimes their possession doesn't work. The reason why Flowey always possesses Papyrus is because Papyrus is stronger than Sans (in health, defense, attack, etc.). Another common target is Papyrus's brother, Sans. The reason for him possessing Sans is because his abilities are much stronger than Papyrus's (Karmic Retribution, dodging, gaster blasters, etc.).
Flowey mostly possesses the brothers, (Sans and Papyrus) due to them being stronger and powerful than most others. Flowey also gets to be sitting at one of their eyes. However, sometimes Sans eye might start glowing and since Flowey is sitting there he gets burned.