The very name Yaksha comes from the phrase they were supposed to have uttered when Brahma created them, "Yakshamam! - We shall protect!" It is therefore not surprising to realize that Kuvera is the guardian of the North Quadrant of the earth and has under him all other Yakshas who did not make it to divine status. Those who did included Laxmi, Ganapati, Hanuman and Kali who all seem to have been local Yaksha deities to begin with before assuming Pan-Indian importance. The fact that the Sapta-Matrikas, the Seven Great Mother Goddesses are always represented in sculpture being protected by Kuber on one side and Ganapati on the other is clear enough indication of their common origins.
In Hindu mythology Kuber is represented as the son of Lord Brahma. He is the ruler over a hidden city in the Himalayas called Alkapuri, which has all the stored up wealth of the Earth. This is close to the abode of Shiva, Kailasha. Kuvera is supposed to be a Shiva- worshipper as well as close friend of the great god hence his name Isasakha - which is just a mythological acknowledgement of the process of assimilation. He happens to be immortal and as a good Yaksha, one of the guardians of the Earth. His half brother was the famous Ravana, who caused him no end of trouble and took away his other kingdom, the fabled city-state of Lanka.
His wife is called Yakshi. In Buddhist mythology she was called Hariti - the stealer, as she had the distressing habit of spiriting away children until Buddha gave her a taste of her own medicine by concealing her child for a while and affecting a cure. Yakshi is also a matrika, who have peculiar ambivalent responsibilities - allowed to plague children till sixteen after which they have to protect them for life.