Pazu and Rin are two minor characters in the On the Way to a Smile novella "Case of Red XIII".
A pair of Nibel bear cub brothers, their mother is killed by a hunter after she attacks a child in the forest. Red XIII sees them inspect the corpse and leads them away to avoid the hunter when he returns. He then cares for the two as they grow, teaching them to hunt and to avoid humans, until they abandon him and go off on their own.
Shortly after, Red XIII finds them engaged with the son of the hunter who killed their mother. The two attack the boy when he attempts to shoot them, but Red XIII intervenes when they try to kill him. He intercepts their attack, and while he sleeps that night, the two are killed by more hunters. Raising the bears and their loss further fuels Red XIII's inner fear of losing those he cares about.