Haru Okumura doesn't join the Phantom Thieves at first especially when she suspects them of only rousing the public for fame and fun when she wants to change her father's heart to save the innocent. 

But when they save her and Morgana from her Fiancé she realizes that they aren't bad people and joins them, who after hearing her story decide to change Okumura's heart to save Haru from her rotten fiancé. 

She had already awoken to a persona but it was weak when she stands up to her father however it realizes it's full potential and she becomes a reliable ally, even though she was raised to have poise and almost a nobility kind of personality when she works as a Phamtom Thief she becomes the complete opposite her weapons are even a grenade launcher and a battleaxe.