Even though Boris is just a freeloader at Gowland's amusement park, they get along quite well most of the time. Boris doesn't work (except in rare cases) which somewhat annoys Gowland, but as long as Boris doesn't cause trouble- whether by getting blood everywhere or by destroying anything- Gowland tolerates his presence. The two argue fairly often, even to the point of drawing guns; this is normal in Wonderland, however, even among good friends.
Their relationship is similar to that of any cat and mouse: Boris is constantly either trying to eat Pierce or is pestering him in some way, terrifying Pierce and resulting in him losing sleep. The two are also very competitive, and when they're not fighting, they get their best work done when trying to outdo the other.
Because Boris is a cat, Vivaldi adores him, for no other reason than because he's a cat. Whenever she sees him, she either pets him or gives him cans of cat food. Boris doesn't eat cat food and isn't very sure about what to do about this, although he doesn't dislike Vivaldi.
Peter hates cats, and consquently despises Boris. Boris doesn't dislike rabbits, but he dislikes Peter. They will try to shoot one another on sight.
Ace seems to not hold any paticular dislike or hate of Boris though doesn't see him in a very good light probably due to the way Boris steals Alice's attention so easily and has been shown to not be against killing him as in the manga adaption Heart No Kuni No Alice Ace attempted to kill him in the forest. Boris respects the way he's incredibly good at fighting and also seems to be a bit afraid of Ace and is a bit cautious around him after deciding to live so he can spend time with Alice.
Boris seems to rather like Julius, while Julius is neutral towards him. If Julius comes to the amusement park, Boris is likely to try to drag the reclusive Clockmaker onto rides... even though Julius is not at all good with amusement park rides.
Boris and Nightmare are friendly with one another but do not interact much until Daiya no Kuni no Alice. In Daiya, Boris and Nightmare both live at the station and Boris helps take care of Nightmare.
The two are on good terms, and, because Boris is a cat, Gray rather likes Boris. They do not interact much, but they get along well. In Daiya no Kuni no Alice, they both live at the Station, so they interact far more and both take care of the sickly Nightmare.
Although Mary's enemy is Blood Dupre, Boris doesn't seem to care and says that it's none of his business on what goes on between them. He sometimes comes to Blood's tea parties.
Boris and Elliot are on good terms. Whenever Boris is allowed to attend tea parties, Elliot and Boris get along fine.
Tweedle Dee & Dum
Boris and the twins are good friends and visit one another often. As part of their friendship, they fight with real weapons, although rarely because they're actually mad at one another; that is how they "play." However, when it comes to Alice, they will more seriously fight if there is jealousy.
Much like every other role holder, Boris does not like Joker, though Joker hold no animosity toward him unless he invades the prison.