Undyne is passionate about everything that she does. She acts quickly on her ideas and is steadfast in her beliefs.  Though she wants to defeat the protagonist, she also tries to be fair and offers an explanation of how Green Mode works.  Asgore taught her the value of pacifism, which is why Undyne sees the protagonist as "a wimpy loser with a big heart" When the protagonist first encounters her, she fights to further Asgore's plan of collecting seven human SOULs but, if the protagonist shows mercy, Undyne just as quickly defends them from Asgore.

Undyne has a passionate, slightly tomboyish voice