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Lets talk about Plumeria and team skull's origin a bit.
I have just visited Plumeria's trailer in oasis. The trailer is parked near old abandoned mart area. Inside there is one female skull grunt/punk girl and an old woman. Fem grunt told you that team skull was once a group surrounding old kahuna, until they was smacked down by Tapu Bulu. The old woman told you Tapu Bulu is capricious and his choice in Kahuna is unpredictable. The old woman is pretty out of place as most NPC surrounding the trailers are punk boys and girls. I therefore assume that old woman is Plumeria is mother/grandmother.
My theory is that the old woman is old Kahuna's wife(or the old Kahuna herself), which make Plumeria his daughter/grandchild. Long ago, old Kahuna make a mistake of allowing a mart to be built on Bulu's land, causing him mad and struck down the place. He also either kill the old Kahuna or strip his title from him/his family, bestowing it on Nanu, who obviously not in line and not really want it. This turn old Kahuna's supporter led by his heir Plumeria to become angry and rebellious at Bulu and by association the Alola tradition overall.
They then later joined with Guzma(who came from Memele island) and his disgrunted homies, and forming what would become team skull. Guzma who is more aggressive and charismatic become the main leader, while Plumeria step back and help taking care of the grunts' welfare.
Also notice that Plumeria specialized in poison type, perfect type to kill Grass/Fairy Bulu. Her main Salazzle is even more fitting in killing him as he is poison/fire.