Hey, it's just hit my head that weiss only possess girl with negative emotion, then it's possible the goddess can only possess girl who have love emotion right?
And you know? Almost all girl that possessed by runaway spirit have something she love (track and field, singing, shogi, keima, etc) make it's possible they are possessed by goddess. I suspect the author set it up like that purposely to give all of them a chance to have goddess.
Diana possess tenri in order to help her. Because tenri's love is big, Diana power come back rather quickly. Apollo possess kanon as runaway spirit, and we know kanon's negative feeling is bigger thus make the weiss stronger and apollo become weak. I suspect vulcan choose tsukiyo because she is beautiful, then vulcan become weaker because tsukiyo's negative feeling.
Why vulcan choose tsukiyo? The fact that apollo and vulcan remember about keima make it a proof that the goddess has her conciousness when they possess someone, just like diana when she possess tenri consciously. My assumption is the goddess choose her own vessel, but become weaker due to vessel's negative feeling (the exception is diana).
Just like keima said, "proof before theory". I create my theory based on fact that I found...
