Definitely the two main characters. The young boy, Takao, is interesting because of his dreams and his growing maturity. Writers can explore his thoughts and feelings as he transitions from a boy to a young man. Yukari, the woman, is also a great character to write about. Her mysteriousness and the reasons behind her actions can be delved into in fanfiction.
Fine is also a popular choice. Her position as a vampire yet having some sympathy towards humans offers a lot of room for exploration. Fanfiction can delve into her internal conflicts, such as her loyalty to her vampire kind and her growing connection with humans.
Circe is a very interesting character for greek fanfiction. She has the power of magic and transformation. Writers can explore how she uses her powers to manipulate those who come to her island. Also, Odysseus is a classic character that is always interesting to write about in new ways. His long journey home can be reimagined, or his interactions with other mythological beings can be expanded upon.
Gerry is definitely one of the most interesting. His unique love for animals and his innocent yet inquisitive nature offer a lot of room for creativity. Writers can create all kinds of scenarios where Gerry discovers new species or has heart - warming interactions with the local wildlife.
Alice is a very interesting character to write about. Her ability to see the future gives so many possibilities for plot twists. Writers can explore how she might use or be haunted by her visions in different situations. Another is Jasper. His past as a Confederate soldier and his struggle with his bloodlust can be delved into deeply.
For me, Sol Star is an interesting character to write about in deadwood fanfiction. He's often in the background but has his own dreams and desires that can be further developed. Then there's E.B. Farnum. His conniving and somewhat pathetic nature can be a source of great comedic or tragic stories. And of course, Wild Bill Hickok. His legend and his short time in Deadwood provide a lot of inspiration for different storylines.
Link is a very interesting character. His complex backstory with the Triforce and his many adventures in Hyrule give a lot of material for fanfiction. You can write about his internal struggles, like when he has to make tough decisions to save his kingdom. Another interesting one is Kirby. His cute appearance but powerful abilities create a contrast. You can write about Kirby exploring new planets and befriending other SSB characters.
In my opinion, the Death Knight can be a very interesting character for quake fanfiction. It has this dark and menacing presence. You could write about its past, like how it became this powerful and feared entity. Another interesting character is the Fiend. You can explore its relationship with other creatures in the Quake world, or how it hunts its prey. And the Vore is also an interesting one. It has this unique and grotesque appearance, and you could write about how it came to be in the Quake universe.
Yato is definitely one of the most interesting. His complex personality, with his past as a war god and his current quest for recognition, gives a lot of material for fanfiction. Hiyori is also great. Her ability to cross between the worlds and her kind - hearted nature make her a fascinating character to explore in different scenarios. And Yukine, with his growth as a shinki and his relationship with Yato, offers a lot of potential for interesting stories.
Elise is a really interesting character to write about. Her grace and skill on the slopes make for great descriptions. Mac is also cool. His laid - back attitude combined with his amazing stunts gives a lot of material for fanfiction. And then there's Zoe. Her bubbly personality and determination can add a lot of charm to a story.
Max Sterling is an interesting character for robotech fanfiction. He's an ace pilot with a cool - headed nature. His relationship with Miriya, a Zentraedi who becomes his wife, is full of potential for stories. Lisa Hayes is also great. Her strong leadership and her evolving feelings for Rick Hunter can be the focus of many interesting plots. Then there's Khyron, the unpredictable Zentraedi who always brings chaos and danger.