Who is Ichigo's Father, Isshin? Though he is initially shown as a normal human, it is later revealed that he once had Shinigami powers, and later he is the only one


   Who knows about Ichigo's true powers, and who tells Ichigo how to train for the last battle against Aizen. 


But the The Lost Substitute Shinigami arc where 


   Ichigo fights Ginjo, it's revealed that Ginjo was the first Substitute Shinigami.


So what is Ichigo's father and why does he know what he knows? Also, why he doesn't seem to have any Shinigami powers, while he knows so much about them?
Isshin Kurosaki used to be a Shinigami of an unknown division and rank. According to bleach.wikia.com:


  About twenty years ago, he ceased being a Shinigami, losing most of his powers.


He still has a shikai, which he uses during the Fake Karakura arc.

Spoiler:


  Ichigo also has Quincy powers that have been implied to be related to his mother. If Ichigo's mother was a Quincy, Isshin may have known.