Taiyang is Zwei's owner. Zwei is very comforting to Taiyang, cheering him up when Taiyang thinks of his former team.
Zwei seems to love Ruby a lot as he is shown affectionately licking her in "Field Trip".
At first, Zwei wants to get along with her, but she tries to avoid him.
Not much interaction happens between them on-screen, but he and Yang have a friendly and playful relationship.
When Oobleck discovers Ruby brought Zwei along in "Search and Destroy", he happily includes the dog as part of the group. They work well together in the subsequent fight against the White Fang, during which Zwei chooses to stay behind with Oobleck when Ruby needs to move ahead.