Well, Allen is frequently presented as a determined fighter. His determination to fight against the Akuma and his connection to the Innocence are important aspects of his character in fanfiction. Alongside that, his relationship with Lenalee is often filled with moments of tenderness. Lenalee is often described as gentle yet fierce. Her family background and her role in the Black Order are important elements that shape her character. And her relationship with Allen is a key factor in many fanfictions, showing how they can bring out the best in each other.
Allen in these fanfictions is typically a complex character. He has his own goals related to the Millennium Earl and the Akuma, but his feelings for Lenalee add another layer to his personality. He might be hesitant in expressing his emotions at times. Lenalee, on the other hand, is very empathetic. She is able to sense Allen's pain and tries to ease it. Their relationship in the fanfiction is usually based on mutual understanding and a growing, unspoken love that gives both of them something to fight for.