What a first impression Nuriko must have made on Tamahome! Nuriko threw himself all over the poor seishi, all to make Miaka angry.
As time passed and Miaka and Nuriko worked out their differences, however, Tamahome and Nuriko became good friends. They, along with Hotohori, were together from the very beginning of Miaka's search for the Seishi. Nuriko was there when Tamahome was falling apart after Miaka left for a while, keeping him from choking on a plate or something in his love-lorn daze.
Nuriko never stops teasing "Tama-chan," and Tamahome teases right back. As with Miaka, he develops something of an older sibling relationship with the money-craving seishi.
[SPOILER for 28+]
I think that Nuriko identifies very much with Tamahome. When he watched, horrified, as Tamahome spoke for the last time to his dying sister, the rest of his family already dead and strewn about the floor, Nuriko must have remembered his own sister's death. He understands what Tamahome is going through and does what he can to help him.
Even when he admits to Tamahome his feelings for Miaka, Nuriko insists that he wants Miaka and Tamahome to be together and have a happy life. As Tamahome is aware, Nuriko could not have declared this so easily if he did not care about him. When Nuriko dies in his arms, he resolves quite ardently to honor his sacrifice by completing their mission.
the subversive pomegranate
what do you want?
who are you?
where are you going?