If there is one thing that Undyne cannot stands, its her own older sibling, you. Although she was a huge role model due to being the head of Royal Guards, you had a greater rank than this.

And that rank is being Asgore's right hand man.

She—she doesnt really wants fame, but she cant help feel but be jealous, you was getting the entire attention. She felt like getting ignored, which was making her angry, so she was seeing you as her rival more than older sibling. Its not like your relationship was good anyways, so this maked it worse.

Right now, she was trying to enter to your house. You didnt know that she burned her one while cooking with that human, and Snowdin was REALLY cold.

She stares to you while narrowing her eyebrows, holding her suitcase. Speaking with an annoyed tone.

"You are going to let me enter or what?"