One inspiring Supergirl story is her fight against powerful villains to protect Metropolis. She uses her super strength, flight, and heat vision to keep the city safe. For example, when a group of alien invaders threatened the city, Supergirl was the first to stand up and fight them off, showing great courage.
Make the threat specific and have clear consequences. Also, build up the tension and make the character delivering the threat seem serious and determined.
Supergirl's exuberance and her willingness to trust can sometimes clash with Batman's more cynical view. However, in the context of a romance in fanfiction, this can be a source of growth for both of them. Batman might start to trust more because of Supergirl, and Supergirl might become more cautious under Batman's influence. Their personalities also play a role in how they show affection. Batman might be more reserved in his displays, while Supergirl could be more open and affectionate, which could lead to some sweet and sometimes awkward moments as they navigate their relationship.