It isn't a mutual dislike so much as a rivalry that only exists in Garmr's mind, because he's a jealous brat that gets mad when Master/You pay attention to other dogs.
The relationship charts for each char feature four people in total: two they like and dislike, and two that like and dislike them. Then there's four slots for potential lovers, so it may be possible to actually pair Andvari/Mori and Garmr/Cusith when Love Quests are available.
There's admittedly a hole in this theory since some characters occupy two slots on another's relationships. Magan likes Nomad but Nomad hates Magan. Plus the Hero is supposed to be available for every character, but I don't know if they take up one of those four slots