I'm a Lydia Martin fan and trust me she's a rare character. There aren't many femslash stories featuring her in TW but the most popular ship is Allison Argent/Lydia Martin. Again, there aren't many het stories featuring her but the most popular ships are Lydia Martin/Jackson Whittemore, Lydia Martin/Stiles Stilinski and Lydia Martin/Aiden. You can cross-check with these. The bottom line is that there aren't many fanworks that feature Lydia Martin as the main character. She is always a secondary character or belongs to a secondary/background ship. That's why she appears in so many fanworks.