As a representative of the “heart” card suit, Hathy is meant to parallel Kris/Frisk as a protagonist with a heart-shaped soul. Because of this, the player can't read her dialogue. Instead, the game narrates her thoughts and actions.
... And yet, like Kris and Frisk, she can still speak. We can't read her dialogue, but other characters (namely Susie) can still hear what she's saying.
(Hathy will never forget what you've done.)
[Susie] In a good way?
(In a bad way.)
[Susie] Oh.
(This dialogue somehow eluded me for seven years.)