is he a good deity according to the puranas?
Yes, as The "Purana"s grant him unquestioned godhood.
Kubera
The "Purana"s and the epics "Mahabharata" and "Ramayana" grant Kubera unquestioned godhood. 
what is the relation between kuber and ravan in mythology Kubera is described as the uncle of Ravana in the "Mahabharata". what is this former one worshipped for? He is worshipped as the god of wealth. what significance it has in other religion? In Jainism, he is the attendant Yaksha of the 19th Tirthankar Mallinath.
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Kubera [SEP] Puranic and epic descriptions: Godhood acquired
The "Purana"s and the epics "Mahabharata" and "Ramayana" grant Kubera unquestioned godhood. Kubera also acquired the status of the "Lord of riches" and the wealthiest Deva. He also becomes a "Loka-pala" ("world protector") and guardian ("dik-pala") of the North direction, although he is also sometimes associated with the East. Kubera's status as a "Loka-pala" and a "dik-pala" is assured in the "Ramayana", but in the "Mahabharata", some lists do not include Kubera. Thus, Kubera is considered a later addition to the original list of "Loka-pala"s, where the gods Agni or Soma appear in his place. This status, the "Ramayana" records, was granted to Kubera by Brahma—the creator-god and father of Pulastya—as a reward for his severe penance.