She could be heartbroken. Tonks is a passionate character. She may initially be in denial, not wanting to believe that Lupin would leave her. She might try to talk to him, convince him that they can overcome any difficulties together.
There are also authors who are really good at character development in their fanfictions. Take 'TonksTales' for example. They create stories where Lupin and Tonks' characters are very true to the original books but also have new and interesting facets added. They might explore Lupin's insecurities in a new light or show Tonks' Metamorphmagus abilities in unique situations. Another great author could be 'RemusAndNymphadora' who writes stories that are often set in different time periods of the 'Harry Potter' world related to Lupin and Tonks.
One interesting fanfiction could be about Tonks and Lupin's relationship during the early days of their acquaintance at the Order of the Phoenix. It might explore how they first noticed each other's unique qualities, like Tonks' Metamorphmagus abilities catching Lupin's attention.
One interesting fanfiction could be about Lupin and Tonks' relationship during the final battle. It might show how they supported each other and fought side by side. Another could be a pre - story of how they first met, perhaps at the Order of the Phoenix headquarters, with Tonks being all clumsy around Lupin which is really cute. There could also be a fanfiction where after Lupin's death, Tonks in a magical way gets to communicate with him one last time to say goodbye properly.
One possible reason could be his self - loathing due to his werewolf condition. He might think he's not good enough for Tonks and that she'd be better off without him, fearing that his lycanthropy could bring danger or a difficult life to her.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of 'Harry Potter' fanfictions, and you can specifically search for ones related to Tonks and Lupin. Just type in their names in the search bar and you'll get a bunch of options.
One great Lupin and Tonks fanfiction is 'The Lupin - Tonks Story' on FanFiction.net. It really delves into their relationship, from their initial meetings to the deeper emotional bonds. The writer captures Tonks' free - spirited nature and Lupin's more reserved personality very well, making their interactions seem natural.
Well, first of all, they have very distinct personalities. Tonks is this fun - loving, shape - shifting witch, and Lupin is the more brooding, intelligent wizard. Their contrast makes for an interesting dynamic. Also, their relationship in the canon is somewhat underdeveloped, so fanfiction writers get to fill in the gaps. They can explore how Tonks first fell in love with Lupin, how Lupin overcame his self - doubt to be with her, and all the little moments in between that we didn't get to see in the books. It's like they are a blank canvas waiting to be painted by creative minds.
FanFiction.net also has a lot of Lupin and Tonks fanfiction. Just use the search bar and type in their names. You might find some hidden gems there. Some authors are really dedicated to exploring their relationship in unique ways on this platform.