Well, in levi and petra fanfiction, a prevalent theme is duty. They have a sense of duty towards their mission and their superiors, and this often clashes with their personal feelings. Another is the theme of regret. Maybe Levi regrets not seeing Petra's feelings sooner, or Petra regrets not being more forward. The theme of courage is also there. They show courage in battle, but also in their personal relationships, like having the courage to admit their feelings. These themes help to make the fanfictions more complex and interesting.
One great Levi and Petra fanfiction is 'A Soldier's Promise'. It beautifully portrays their relationship within the military setting. Another is 'Petra's Hope', which delves deep into Petra's feelings for Levi. There's also 'Levi and Petra: Unspoken Bonds' that shows their unspoken connection in a very touching way.
The essence of Petra Collins' love story lies in its ability to capture the complexity of human connection. It shows the ups and downs, the joys and pains of love in a very real way. There's a depth to the characters' feelings that makes it truly engaging.