Well, AO3 is the go - to place. It has a great search function that allows you to specifically look for Armin x Jean fanfiction from 'Attack on Titan'. You can also try Wattpad, although it has a more mixed bag of content. Sometimes, Tumblr might also have links or short fanfictions related to this pairing if you search through the relevant tags.
Archive of Our Own is definitely the best bet. It not only has a wide range of 'Attack on Titan Armin x Jean fanfiction' but also has a community that often reviews and recommends good stories. Another option could be Reddit. There are some subreddits dedicated to 'Attack on Titan' fanfiction where you might be able to find some great Armin x Jean stories. Just make sure to follow the subreddit rules.
You can try fanfiction websites like Archive of Our Own (AO3). There are a large number of Attack on Titan fanfics there, including ones about Jean and Armin. Just use the search function and type in 'Jean x Armin' within the Attack on Titan fandom section.
One great 'Attack on Titan Armin x Jean fanfiction' is 'Bonds Beyond the Walls'. It beautifully explores their relationship as they face the challenges of the Titan world together. The story delves deep into their mutual respect and how it evolves into a strong bond. Another is 'Dawn of a New Alliance' which shows them working side by side in a post - Titan - threat world, where their friendship takes on new dimensions.
Some fanfics might explore a romantic relationship between them in a more secretive or forbidden way. Given the military setting and the overarching plot of Attack on Titan, their relationship might be something they have to hide. This adds an element of tension and drama to the story, as they navigate their feelings while also dealing with the threats of the Titans and the politics within the walls.
Fanfiction.net might also be a good place. Just use the search function and type in 'Armin x Jean'. However, the quality can vary a lot, so you might need to skim through a few to find the really good ones. Sometimes, Tumblr can also have some great fanfiction recs or even short fanfics posted directly.
Fanfiction.net could also be a place to look. There are many different categories and pairings available there. However, the quality and content can vary greatly. Sometimes you might need to dig a bit deeper by going through different tags and sections related to the series that Jean and Armin are from to find the yaoi fanfic you're looking for.
One popular theme is the growth of their relationship. Starting from distrust or indifference and evolving into a deep friendship or something more. Another theme could be them facing challenges together, like a particularly difficult Titan attack where their combined skills are put to the test. Also, the theme of sacrifice might be present, where one is willing to sacrifice for the other in a life - or - death situation.
One popular trope is the 'enemies to lovers' dynamic. Given their positions as former enemies in the main story, it's common for fanfictions to explore how they transition from hostility to love. Another trope is the 'hurt/comfort' where, for example, Annie might be injured or in distress, and Armin is there to comfort her, which deepens their bond.
One popular theme is their friendship evolution. As they grow up in the dangerous world of Attack on Titan, their bond changes and deepens. Another theme could be sacrifice. How far would Eren or Armin go to protect each other? And there's also the theme of trust. Given the complex and often deadly situations they face, trust between them is crucial and can be a major focus in fanfiction.
There's also 'Armin and Mikasa: A New Chapter' which focuses on their emotional connection. It shows how their friendship might develop into something more. In this fanfiction, they are often seen having deep conversations and relying on one another in difficult situations.
Some Reddit communities dedicated to 'Attack on Titan' also share fanfiction links. For example, r/ShingekiNoKyojin might have posts about fanfiction with a Titan reader. You can interact with other fans there and get recommendations directly from them.