Summary
('Undyne', as heard in Spear of Justice)
Undyne's theme.
It's heard in most every song relating to her, and as such is often used alongside 'Ruins'.
In Dating Tense! and Undyne, Undyne also has a different, more 'spooky' melody:
('Undyne (Tense)' overtop of 'Undyne', as heard in Undyne)