They might create false stories or alibis. Say if they go on a date, they would tell their friends they are doing something else separately. Also, they could be careful about leaving any evidence of their relationship, like not leaving love notes where their friends could find them.
They could be relying on their friends' lack of suspicion. Since no one would expect them to be married, they can act normal most of the time. But when it comes to certain situations like family events related to their marriage, they would come up with really good excuses not to bring their friends along, so as to keep the secret.
They might use magic to their advantage. Natsu could use his fire magic to create distractions when there's a chance their secret could be revealed. For example, if someone is about to walk in on them sharing a tender moment, he could create a small 'accidental' fire to divert attention. Lucy, on the other hand, could use her celestial spirits to help them communicate privately. She could summon a spirit in a secluded area where they can talk without being overheard.
To keep their marriage a secret, they would have had to be very careful. They could have met in secluded places like an abandoned forest or a hidden cave. When communicating with others, they would be extra cautious not to let any hints slip. Natsu might have pretended to be just a friend to Yukino when around others, and Yukino would do the same. They might also have had a secret code or signal to let each other know when it was safe to be together.
Perhaps they just want to enjoy the early days of their marriage without the scrutiny of the outside world. They want to have a normal relationship for a while, just the two of them. They can learn about each other in a more private and intimate setting. It's like a honeymoon phase that they don't want to be interrupted by the media or public opinions.
One possible plotline could be how they manage to keep their relationship a secret from their friends and classmates. They might have to come up with elaborate excuses for why they're spending time together or being overly protective of each other in a way that doesn't seem too obvious.
Ochaco Uraraka could be a main supporting character. She's Izuku's friend and might notice some changes in his behavior due to the secret relationship. Her reactions and how she might accidentally get closer to uncovering the secret could add a lot of drama.
She probably uses magic. For example, she could use a spell to mute any sound coming from her music player when someone gets too close. Or she might have a charmed diary where she writes about her love for the singer and it can only be opened by her magic signature, so no one else can find out.
A great way to develop their relationship is through shared traumas. Maybe they both had a difficult experience during a villain attack. They could open up to each other about their fears and insecurities during that time. This vulnerability would bring them closer together. Additionally, they could participate in a hero competition together. As they face tough opponents, they encourage each other and celebrate their victories, which deepens their bond.
They might be afraid of how their families will react. The Weasleys have their own ideas about who Fred should be with, and Hermione's parents, while muggles, might not understand a hasty and secret marriage. Also, there could be threats from dark forces. If the wrong people find out about their union, it could put them in danger, so secrecy is key.
Good acting skills are essential. They need to be able to change their demeanor depending on which 'life' they are living. When in their public life, they act normal, and when in their secret life, they can switch to a different personality. They also need to be cautious about who they interact with. Not getting too close to people who might discover their secret is important. Another aspect is controlling their digital footprint. In modern times, if they are living a double life, they have to be careful about what they post on social media, as it can easily give away their true identity or activities.
One possible aspect to consider in fem Izuku and Katsuki fanfiction is their character dynamics. In many stories, fem Izuku might bring a different kind of vulnerability compared to the male version, while Katsuki could still be his brash self. Their relationship might develop in new ways, perhaps with more emotional depth due to the different gender portrayal. For example, fem Izuku might be more likely to show her emotions openly, which could either draw Katsuki closer or push him away at first as he tries to deal with his own feelings in this new context.