Combo of story surrounding the character the player is invested in is the kryptonite crack.
My game (Folke, available as PWYW here on itch) has within the Character Creation Rules a mechanic to create 3 characters, all intertwined, one of which is played, the others are invested in the story and used as NPCs (which are back-up characters in case of character death).
Having one of your characters kidnapped by the BBEG is an invested pull.
Having one of your characters influenced by the BBEG and starting to work against the extended party, with avenues for the player to invoke a redemption arc for their own character, unlocks huge roleplay investment.
J
(Author of Folke)