Significant Impact
Amaterasu: Hiei didn't care that she was a god. From his perspective, she was threatening Kurikara, and that was enough for him to step in.
Byakko: Hiei's first impression was unfavorable, but Byakko wasn't terrible by the end of his stay in Landel's. Loved humans a wee bit too much.
Diva:
Edward Elric: Hiei's roommate. For the most part, they don't get along. They're getting better since that sedated cursing contest though. And Ed can make swords, which Hiei finds very useful.
Keman: Kurikara's apprentice, which is the main reason Hiei has anything to do with him. But since he is Kurikara's apprentice, Hiei figures he can help the dragon avoid being stupid at least.
Kurama: Former patient, Kurama is from Hiei's world. There's a level of trust here, and one of understanding since they've worked with each other for awhile, and because both are perceptive. Hiei doesn't like how Kurama will laugh even when he answers his questions, but at the same time he wouldn't go to anyone else for those answers. Notably, in Landel's, Kurama's choice to pretend to be human pissed Hiei off to no end.
Kurikara: Currently, Kurikara is the person Hiei clicks with most in Landel's. They have a standing agreement to beat the hell out of each other when one of them does something stupid. Their personalities just seem to mesh, and Hiei is both intrigued and annoyed by how much Kurikara cares. Won't admit it, of course. They understand each other despite not knowing each other for very long.
Waka: If he had stayed, Waka and Hiei might have slowly begun to get along, especially after their truce. Waka was weird, but tolerable.
Others Met (well or otherwise)
Agrilla
Artemis
Citan
Danny Fenton
Hitsugaya
Kratos
Nadine
Sai
SubZero