Who Yama is
Yama is the king of the dead, and in the Rigveda that is good news. He is the reason the dead have somewhere to go.
Only three hymns of the Rigveda are his own (10.14, 10.135 and 10.154), with a fourth that is a conversation between him and his twin sister Yami (10.10). His name comes up about fifty times, nearly always in the first and the tenth books. The poets were more interested in this life than the next. What little they say about him is steady and warm.
He is the son of Vivasvat, and he is called a king, not a god. One hymn calls him the only mortal. He chose to die, the poets say, and gave up his own body, and then he travelled to the heights and found a path that nobody had walked before. At the end of it he found a place to live, a pasture that can never be taken away. The Atharvaveda says it outright: he was the first of mortals to die.
Everyone who has died since has taken his road. The funeral hymn tells the dead man to go by the old paths, to hurry past two four-eyed dogs, and to join the fathers who are glad in Yama's company. There he sits under a tree with fine leaves, drinking with the gods, while a flute is played. Nobody is judged at that table. People press Soma for him, offer butter, and ask him for long life.
The dogs are his messengers, and they also walk among the living, watching. An owl or a pigeon can be his messenger too. One poet speaks of the fetter of Yama, and of death as his path, so there was some fear in the picture even then. But the Vedic Yama has no hell and no ledger of sins.
His name means twin. Iran's old scriptures know him as Yima, son of Vivanhvant, a king of a golden age, which tells scholars that he is older than the Veda itself.
Yama was the first to find the way for us. This pasture can never be taken from us.