Quick transmigration : The Extra is invincible.

Every novel has a extra that acts as background just to fill the space. She, who acts like a mole visits every world to mess up the plot.
World 1:
High school male lead who is the topper x introvert female student who is always the second
world 2:
Domineering CEO x his ex crazy fiancée
world 3:
Poor Student x friend's terminally ill sister
world 4:
Cold Professor x one of his students
world 5:
80s Soldier x Military doctor
world 6:
New actor x actress in same show