Open skill on UNITS go by agility. So things like Bringer and Sakura will always go before Wisp since they are faster. If the units have the same agility then it becomes a toss up. Also Grim open skills always go before Unit open skills.
"While Retear in his previous state was certainly powerful, we felt that his skill set was not particularly satisfying to use..." A.K.A. he was too Refess for Refess.