In this scenario, Sakura would be horrified. Her view of Itachi would turn from perhaps a neutral or even positive view (if she had any such view in the fanfiction world) to one of pure hatred. Their relationship would become one of enmity. Itachi using her as a shield would be a huge violation of trust and basic human decency, so she might even try to expose his actions to the other characters, causing a rift in the overall story's character dynamics.
One motive could be that Itachi was trying to protect a greater secret or a more important person. He might have calculated that sacrificing Sakura's safety in that moment was worth it to keep something else safe. Or perhaps he had some sort of vendetta against Sakura or those associated with her. He could be using this as a way to get back at them. However, this is a very dark and immoral motive, but in the context of a fanfiction where such a scenario is presented, it's a possible explanation.
Sakura's amusement could actually make Shisui more likely to continue annoying Itachi. Since she finds it funny, Shisui might see it as a sign that his actions are acceptable and entertaining. This could potentially strain the relationship between Shisui and Itachi further if Itachi doesn't like being the target of Shisui's annoyance.
In a vampire fanfiction, their relationship might be a tumultuous one. There could be many obstacles in their way. For instance, the vampire society they live in might not accept their relationship. But through their shared experiences, like fighting against a common enemy or helping other vampires in need, they could grow closer and form a strong bond that defies the odds.
It can be shown through Itachi's actions. For example, he might not allow other characters to get too close to Sakura. He could give warning looks or subtly use his power to keep others at bay.
They could start as rivals in academics or sports. For example, if there's a science competition in the highschool, both Itachi and Sakura are determined to win. However, as they prepare, they start to respect each other's intelligence and hard work. This mutual respect could be the first step in developing a relationship.
As the story progresses, they may find common ground. Maybe they both have a goal related to the Akatsuki's activities that aligns them. Itachi's past and his reasons for being in the Akatsuki could be revealed to Sakura, and she might start to see him in a different light. For instance, if she learns about his true motives to protect the village, she could develop a new - found respect for him. This could lead to a deeper connection and their relationship could evolve from mere acquaintances to friends or even something more in some fanfictions.
They could start as study partners. Itachi helps Sakura with more difficult subjects like math or history. Through these study sessions, they get to know each other's personalities better. Sakura might notice how gentle Itachi is despite his cold exterior. This could lead to them having lunch together regularly, and then eventually going on outings outside of school, like to the park or the library.
Their relationship could start as a reluctant partnership. Itachi is used to working alone, and Sakura is initially in awe of Itachi's power. But as they face the challenges of the time - travel situation together, they begin to trust each other. For example, when dealing with threats in the past, they rely on each other's skills. Sakura's medical ninjutsu saves Itachi from a near - fatal wound, and Itachi's combat abilities protect Sakura from dangerous enemies.
There could also be a more friendly and equal - footing relationship. They might bond over shared interests outside of combat, like a love for a certain type of flower in the village or a particular kind of food. This would bring them closer together on a personal level. Itachi, who is usually reserved, might open up more around Sakura and Shisui, and they could form a tight - knit group of friends who support each other through the difficulties in the shinobi world.
Their relationship could start as a rivalry. Sakura is known for her tenacity, and Itachi is highly intelligent. They might compete in class rankings or in some school sports. But during this rivalry, they start to respect each other's abilities. For example, Sakura might notice how Itachi is always calm under pressure during a track and field event. And Itachi might see how Sakura never gives up even when she's behind in a test score. This respect could then turn into friendship as they start to study together or train together. Eventually, it could lead to a romantic relationship as they realize they have more in common than they thought.