Yes, it could be. There are many characters in fictional works related to Harry, and Lilian might be one of them if it is within the context of that particular 'Harry Fantastic Fiction'.
Lena Luthor does appear in the comics. Her character has evolved and played significant roles in various story arcs. Sometimes her actions and decisions have a big impact on the overall plot.
Princess Lilian's love story began when she met Prince Bertil. Their love grew over time, but they had to deal with the fact that the royal family had certain expectations and regulations regarding marriages. Bertil was a prince, and Lilian was not from a royal background initially. However, their love was strong. They had to wait for a long time and face public scrutiny. But in the end, their love triumphed, and they got to be together, and Lilian made her mark in the Swedish royal family.
If Lilian is a character in the 'Harry Fantastic Fiction', her significance could be related to the plot. Maybe she is a friend or foe of Harry, or she might hold some important information or power.