In Twilight fanfiction, when a major's mate is in danger, they would experience a strong emotional turmoil. This could translate into a very focused and fierce reaction. They might use their influence or status within the Twilight world to gather resources or help. Their reaction would be centered around ensuring the safety of their mate at all costs, and they would not rest until the danger has passed.
Well, usually the major would sense that their mate is in danger through some kind of bond. Then they would drop everything and rush to the rescue. In the Twilight universe, the major might have to face other supernatural beings who are causing the danger. It could be a fight for survival not just for the mate but also for their relationship. And the whole situation could be a turning point in the story, leading to new character developments.
They are often very possessive. Since they are powerful and used to getting what they want, once they find their mate, they will do everything to keep the reader close. For example, Aro might be very intrigued and want to study the reader if they have something unique, while Caius could be more aggressive in claiming the reader as his.
She might be devastated. Bella was deeply in love with Edward, so his leaving would be a huge blow. She could cry for days and feel like her world has ended.
He might be initially in shock. Harry could stand there, unable to believe what has just happened. Then he might become angry, lashing out at his mate or the situation in general. He could say things he doesn't really mean in the heat of the moment.
Kagome would most likely be very angry. After all, being kidnapped against her will is a huge violation. She would probably yell at Sesshomaru and demand to be released immediately.
After the initial shock and sadness, Twilight often becomes determined. She decides that she will not let the betrayal break her. In a story where she is abandoned in a dangerous realm, she uses her magic and wits to find her way back and also to expose those who betrayed her, showing that she is stronger than they thought.
Bella might go into a deep depression. She was so in love with Edward, and his death could shatter her world. She could become withdrawn and isolate herself from her friends and family.
Edward would likely be in a state of panic. He loves Bella deeply, so he'd probably do everything in his power to save her. He might search for a cure or a way to reverse whatever is causing her to die.
However, some human Cullens might be a bit perturbed. If a fanfiction misrepresents them or their family values in a negative way, they could feel a bit of annoyance or disappointment. For example, if a fanfiction portrays them as being cruel for no reason.
Harry might be deeply hurt. He may initially be in denial, not believing that Severus is rejecting him. After all, he may have felt a strong connection. So, he could try to reach out to Severus multiple times, hoping to change his mind.