A prevalent theme is adventure. The stories usually involve Kasumi and the OC going on various quests or facing dangerous opponents. This gives the opportunity for exciting fight scenes and exploration of different areas in the DOA universe. Also, the theme of loyalty often comes up. The OC may prove their loyalty to Kasumi or vice versa during difficult situations.
One great kasumi doa and oc fanfic is 'Kasumi's New Adventure'. It has an interesting plot where the OC (original character) teams up with Kasumi to face various challenges in the DOA universe. The story is full of action and also delves into the relationship between the two characters.
Kasumi is likely a character created specifically for this fanfiction. She's probably a ninja with a strong connection to the 7 swordsmen. Maybe she's a new recruit or someone with a special ability that makes her relevant to the group.
The character could interact with Kasumi by initially being a bit of a mystery to her. For example, it could be a Pokemon that shows up at just the right time to help her out of a tight spot during a battle or in a dangerous situation in the wild. Then, as Kasumi tries to figure out more about this strange Pokemon, their relationship develops.
I'm not entirely sure specifically what 'doa adult fan fiction' is about as the 'doa' could refer to different things like 'Dead or Alive' (a game series). Fan fiction in general is fictional writing by fans based on existing works. But without more context, it's hard to say precisely what this particular 'doa adult fan fiction' involves.
For an OC in DOA fanfiction to be great, they should have well-defined powers or skills that are balanced and don't overshadow the existing characters. Their interactions with the canon characters should be believable and add depth to the story. Also, a distinct visual design helps them stand out.
Yes, there are quite a few. You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Ranma and Kasumi love' and you'll find a variety of stories with different takes on their relationship.
One interesting doa fan fiction could be about the characters' backstories before the main events of the game. For example, a story exploring Kasumi's training days in her village.