Yato[]
Yukine is Yato's regalia. Yukine finds that Yato is creepy, annoying, and too self-centered. But when Yato praised and became very surprised for Yukine's skills, he became very happy even though he didn't show it, and he later started becoming a little friendlier with Yato. However, after meeting Nora, Yukine felt that he wasn't really special to Yato, so this caused him to do bad things, etc stealing things; stinging Yato several times. At one point, Yukine didn't want to be Yato's shinki anymore and wanted to work for Tenjin, but he refused, saying that Yukine would turn into a Nora if he gave Yukine a name.
But Yato refused to let go of Yukine until Daikoku stepped up. Yukine had stung Yato so much that he was almost about to die, and an ablution was needed. Yukine was given his "punishment", where he committed all the things he had done. When Yukine was about to turn into an Ayakashi, Hiyori commented that Yato had told things to Yukine, like a father would, which deeply touched Yukine.
After the ablution and after Hiyori tells Yato to treat Yukine like a human being with feelings, their relationship strengthens, despite Yukine still finding Yato to be annoying. Later in the series, it is shown that Yukine cares deeply about Yato, which is seen that he sacrificed himself from Bishamon's attack (which turned him into a hafuri, the most loyal shinki). He has also said that he is willing to sacrifice his name for Yato anytime and he is also willing to get stronger, so he can protect Yato. Yato also said that they are quite alike, with both of them having their "father issues".
After Yato told him to light his way to becoming a God of Fortune, Yukine agreed and said that as his exemplary, he's there for him and would make sure that he would never kill anyone again. When Kazuma betrays Yukine's trust and becomes an enemy in his eyes, he also doesn't want Yato to see Kazuma, saying that his master comes first, which indicates that he worries about Yato's safety.
Yato has also said that he fell for Yukine "at first sight", which is why he didn't want to give up on Yukine. He sees Yukine as "his one and only" as well as his pride.
Hiyori Iki[]
At first, it seemed like Hiyori is a possible love-interest for Yukine but later simmers down into a close friendship. Yukine views Hiyori as a valuable friend and someone he can rely on. Yukine also borrows Hiyori's old textbooks and workbooks from time to time and Hiyori enjoys teaching Yukine about various subjects. However, when it comes down to school work Yukine thinks that Hiyori is a little strict.
Although it is shown less in Noragami Aragato, there may be a possible chance his crush on Hiyori may be still there or least a small attraction for her. This is shown when Hiyori remembers Yato and the others after a month again, she immediately hugs Yukine, in which he blushes and nosebleeds.
Hiyori also played a big role in his ablution, giving Yukine words of encouragement which stopped him turning into an Ayakashi, and Yukine is deeply grateful to her for this. After this, Yukine started viewing her as a much closer and more important person to him. Yukine is shown to worry about Hiyori's well being since she is very important to him and Yato. Yukine is seen to worry about her well-being and doesn't want her to do dangerous things since she is very important to Yato and him, and he is also seen to worry that she will forget about him and Yato one day.
Kazuma[]
While Yato was away on his father's orders, Yukine used that time to be trained by Kazuma on Regalia skills. During this time the two grew to be rather close, more like friends than mentor and apprentice. Their relationship helped to calm the last resentment between Bishamon and Yato. However, when Kazuma attempted to force the Sorcerer's location out of Yukine, it made Yukine very angry, causing him to act much colder towards Kazuma and ultimately destroying their friendship. He even went as far as to tell Yato to stay away from Kazuma.
Suzuha[]
When Yukine first met Suzuha, they became friends almost immediately, even though their masters were adversaries. Within the days after Yukine would run to visit his friend as soon as he was done working with Yato. When Suzuha died, Yukine was extremely depressed for a few days after that, nearly reverting back to his original personality.
The Stray[]
From the beginning, Yukine was jealous of Nora. He felt as though, since she was Yato's regalia for a longer period of time, and knew more things about him, that Yato needed her more than he needed Yukine. This caused some strife between him and Yato at first, however, soon the resentment was only directed towards Nora and not Yato. When Yato released Nora, Yukine felt very relieved. He was shocked when she kissed him in Chapter 74 seeming that she may be developed feelings for him. In Chapter 75, it was revealed that Yukine accidentally stung Yato from Nora's kiss and was given an ablution for it. After confessing his sin, Yato forbid Yukine to reciprocate any feelings. After Father throws her away, she turns to Yukine for help, has also had her name revoked from all the other gods. Currently, Yukine is providing her shelter in the home of Kofuku and Daikoku, without their knowledge. During this time, things have softened up a bit between the two, with Yukine getting upset when Nora says learning is pointless, and Nora getting upset about Yukine wanting to learn history, (because she knew the history of her death) and that history was dangerous to learn. Yukine also gets mad at, and confronted Fujisaki, telling him off for throwing her away.