You can modify Sims' relationships by using the somewhat long modifyrelationship yoursimfirst yoursimlast targetsimfirst targetsimlast x relationshiptype. Relationshiptype is either LTR_Friendship_main or LTR_Romance_Main. With Cats and Dogs, you use LTR_SimtoPet_Friendship_Main, which won't work if the pet has no last name. X is the percentage of the bar, either full, half, or half into the red (negative numbers). While there are many different relationship names, these are simply combinations of varying amounts of friendship/romance (either positive or negative). Learn more about Romance in The Sims 4 here.
- modifyrelationship Bella Goth Mortimer Goth 100 LTR_Friendship_main - Bella and Mortimer are now best friends
- modifyrelationship Bella Goth Fluffy Goth 100 LTR_SimtoPet_Friendship_main - Make Bella friends with her pet if playing Cats and Dogs.
- modifyrelationship Bella Goth Mortimer Goth -100 LTR_Friendship_main - this will make Bella and Mortifer go all the way to enemies
- modifyrelationship Bella Goth Mortimer Goth 100 LTR_Romance_main - They're now soulmates if they have a high friendship level as well
- modifyrelationship Bella Goth Mortimer Goth -100 LTR_Romance_main - Drain their romance bar all the way to red