Alphys is the is the 53rd track in the Inverted Fate Soundtrack. It is a character theme for Captain Alphys, and first plays during Frisk and PAPYRUS' first encounter with her in Waterfall, where she hears the sound of them moving in the tall grass and draws her crossbow, scoping out for trespassers below the cliff.
It is the equivalent of Undyne from UNDERTALE.