One common theme is the 'forbidden love' aspect. Since Tails is from the land and Marine is from the sea, their different habitats can be seen as a barrier, like in Romeo and Juliet. Another theme is discovery. They discover new things about each other's worlds and themselves through their relationship.
Adventure is also prevalent. They often go on exciting journeys together, like exploring new islands or searching for lost treasures. These adventures allow for character growth and development.
One common theme is friendship. Since Tails and Marine are from different worlds, their growing friendship is often explored. Another theme is adventure. They usually embark on some sort of journey together.
Another theme is the power of nature. The ocean can be both a beautiful and a dangerous force in marine love stories. It might bring the lovers together, like when they meet on a beach during a storm, or it can test their love, such as when a character is lost at sea. In 'The Perfect Storm' - related love stories, this is quite evident.
A common theme is the exploration of their relationship development. It could start from a chance encounter and gradually grow into deep love.
Friendship - to - love is also quite prevalent. They start as best friends, like they are in the original Sonic series, and then gradually develop romantic feelings. This theme plays on the deep bond they already have and how it can transform into something more.
One common theme could be the overcoming of differences. Since Shadow is often seen as the brooding, dark character and Tails is the intelligent and more innocent one, stories might focus on how they bridge that gap. Another theme could be the power of friendship turning into love over time.
One common theme could be redemption. If Fiona has a troubled past, Tails could be the one to help her see the good in herself. Another theme might be growth. Through their relationship, both Tails and Fiona could grow as characters, learning from each other's strengths and weaknesses.
One common theme is the contrast between the two different worlds. In most of these stories, there is the world of the sea and the world of the land. The characters from these different worlds often face challenges in their love due to cultural or physical differences. For example, in 'Nagi - Asu: A Lull in the Sea', the sea - people have different traditions and ways of life compared to the land - dwellers.
One common theme is adventure. Ash is often portrayed going on exciting journeys in the marine world. Another theme is friendship. He forms bonds with marine creatures or other characters in these stories.
I'm not sure if it's still available, but 'The Tale of Tails and Marine' was quite a hit. It was a bit different as it incorporated elements of time - travel. Tails somehow travels back in time and meets a younger Marine, and their relationship develops in a very unique and unexpected way.