Robin Sena
Sakaki and Robin seem to be at fairly good terms with each other. Sakaki was one of the first people to befriend Robin, having only known what it was like to be new not too long before. At one point in the series, Sakaki becomes overwhelmed by a witch hunt, and is comforted by Robin.
Amon
Amon treats just about everyone with the same attitude: very coldly. Sakaki is no exception. When Sakaki gets into some sort of trouble, Amon is usually steps in to assist Haruto at the climax of the hunt. He sometimes even tells him what to do and what not to do.
Karasuma Miho
These two are usually paired together, while Miho being the one with more experience (and higher hunting class). They seem to get along very well, and even share the same opinnions in certain matters.
Doujima Yurika
Unlike Sakaki, Yurika doesn't seem to take her work seriously because of her so-called "connections". Sakaki is somewhat envious her since she could laze around without risking her job. Although, I'm sure he does not hold anything serious against her. Towards the latter half of the series, Sakaki offers to take her, having a prepared extra helmet. They're sometimes parterned together on missions, the most crucial ones being towards the end.
Michael Lee
The two seem to be friendly with each other, with Michael being the only person on the team who has the privilage to call him by his given name, Haruto, despite being younger than the hunter. Suprisingly, Haruto doesn't seem to mind. After Sakaki becomes injured, he remains at headquarters with Michael for a short period of time.
Kosaka Shintarou
If Sakaki gets into any trouble, he's most likely to answer to Kosaka. Any kind of communication between them usually involves Kosaka scolding Sakaki for his screw ups, and suquentially followed by Sakaki becoming irritated with him.
Hattori Shouhei
Sakaki is always unsually annoyed by him.