After Mad Rat dies, the Rat God gives him the opportunity of reliving his last day on Earth and motivates him into pursuing his goal. After Mad Rat finishes his goal, the Rat God expresses a little sadness over Mad Rat's inevitable death.
You can have Mad Rat express regret on having to kill his (admittedly zombified) rat cagemates. The Rat God then admires the Mad Rat's noble heart. Either way, Heart still holds Mad Rat's cagemates in regard.
Heart, despite not being able to see the same things Mad Rat sees, does support Mad Rat's goal of revenge. At the same time, Heart frequently tries to convince Mad Rat to find other ways of living his last day on Earth while expressing concern of how not everything is the way things seem or Mad Rat claims.
Once Heart reveals that Mad Rat was just hallucinating his adventures (including killing the scientist), Heart becomes a lot more outwardly supportive in getting Mad Rat to the laboratory, even hugging him. Mad Rat gets surprised at this sudden change.
Mad Rat: "Why are you suddenly so supportive?" Heart: "Well, I AM your heart, after all."
A little girl rescues Mad Rat (and Heart) before a cat can eat them.
Though this was out of pity, Mad Rat rewinds time and averts the death of a cat. In turn, the cat helps Mad Rat out of the sewers, transforming from an instant-death obstacle with his claws to using his paws and head to help Mad Rat close gaps he normally wouldn't be able to make! Heart even relays the cat's gratitude to Mad Rat. He doesn't stop there, though. Once he discovers the death of the girl who rescued him from being eaten by the black cat, Mad Rat rewinds time and stops her from getting hit by a truck. In return, the girl gives Mad Rat real cheese. In light of this, Mad Rat wonders if he could have accepted his death had he been able to talk with the scientist... the same scientist that killed Mad Rat.
Once Mad Rat meets the real scientist, Mad Rat can spare the scientist. If so, Mad Rat explains that he had already forgiven the doctor's sins. Heart congratulates Mad Rat on this decision. Helping matters is if you answered in the beginning of the game that the rat should live.
After Mad Rat leaves his mark on the world, the two send off their lives by seeing the moon and bid farewell. Shame about Rat God undoing it all...
When Mad Rat finds out that Heart was actually Mad Rat's failed transplanted heart, Mad Rat insists on Heart fleeing the transplant, even though Mad Rat would die.
After beating Rat God, Mad Rat learns that Heart was actually a cat, which was why Heart never wanted to talk about it. Mad Rat responds by saying it never mattered, then hugs him before sending Heart on his way because Mad Rat does not want to have Heart see him die. After the credits, though, Heart uses his time powers to come back and carry Mad Rat with him so that the two can leave their mark on the world and fulfill both of their wishes.
Heart is reluctant to let Mad Rat see him while alive, since Mad Rat has repeatedly said he hates cats. What does Mad Rat do upon the reveal? Hug his companion and tell him this:
"What you were doesn't matter! You're still my heart."
