Well, the reader could have a very strong reaction. They might be angry, feeling that Sasuke has no right to take them against their will. So they could be constantly looking for ways to escape and get back to their normal life. But also, depending on how the author writes it, the reader could be secretly intrigued by Sasuke. Maybe they have always been a bit fascinated by him from afar, and now that they are in close proximity, they have mixed feelings. This could lead to some internal conflict within the reader as they struggle between their fear and anger on one hand and this strange new feeling of attraction or at least interest on the other.
At the beginning, the reader's reaction could be one of confusion. They may not understand why Sasuke, of all people, would kidnap them. They could start by asking a lot of questions, trying to figure out his motives. If they are brave, they might try to reason with him, but if they are more on the timid side, they could be cowering in fear, hoping that someone will come to their rescue. As time passes, they might start to develop a sense of curiosity about Sasuke and his actions, which could lead to a change in their reaction from pure fear to a more complex mix of emotions.
In some cases, the reader could be in a state of shock and disbelief. They might not believe that this is really happening. But as they come to terms with the situation, they could start to form a plan. Maybe they try to appeal to Loki's better nature, if they believe he has one, by talking about his past actions and relationships that show his capacity for good.
At first, Sakura might be terrified. After all, being kidnapped is a scary situation. But then, depending on her character development in the fanfiction, she could start to try and reason with Sasuke. She might ask him why he's doing this and try to get him to see the error of his ways.
At first, Hinata's reaction might be one of shock and disbelief. She has probably never expected Sasuke to do such a thing. However, she is a strong - willed character deep down. So, she might start to plan her escape. She could use her knowledge of the ninja world and her own family's techniques. She might also try to find out if Sasuke has any allies or if there are other forces at work. She could try to turn the situation around and make Sasuke realize that what he is doing is wrong by appealing to his better nature or by using her own skills to outwit him.
Well, perhaps the plot of the fanfiction where Sasuke kidnaps the reader could involve a misunderstanding. Sasuke might think the reader is working for his enemies when in fact they are not. As for the story development, it could lead to the reader gradually understanding Sasuke's past traumas and reasons for his actions, and in the end, they might even help him heal some of his emotional wounds. And there could be a lot of action scenes where the reader tries to escape but then realizes that Sasuke is not really a bad guy in this situation.
He could also be initially confused. Sasuke has known Sakura for a long time, and this unexpected act from her would leave him wondering about her motives. He may hold back his anger for a while to hear her out and understand why she did such a thing.
A kid reader might be really excited. They could be drawn in by the cool characters and the unique setting of the Undertale world in a dreamy context.
At first, Loki could be a bit stunned. He'd take a moment to process the news. Then, his protective instincts would kick in. He might start making plans right away, like ensuring the safety of the reader and the unborn child. He could also be a bit in awe, realizing that he is about to be a father and thinking about all the new experiences that come with it. He'd likely be very tender towards the reader from that moment on, always making sure she's comfortable and has everything she needs.
Naruto would probably be very confused at first. He trusts Sasuke, even if it's a Fem Sasuke in this fanfiction, and he wouldn't expect to be kidnapped by her. So, his initial reaction might be to try and talk to her, asking her why she's doing this.
Naruko herself would probably be shocked at first, but then she would start trying to reason with Sasuke. She would ask him why he's doing this and try to make him see that whatever he's thinking is wrong. She's a strong - willed character and wouldn't just sit there passively.
Naruto would probably be really angry at first. He's a free - spirited person who doesn't like being forced into things. So he'd likely try to fight back right away, yelling at Sasuke about what he's doing.