Youko Kurama is a five-tailed fox demon who was a fugitive from Makai. He was a thief back then, a good one. One day, he was being hunted and he was seriously injured; his situation was very hostile. Fortunately, he found a way to survive. He turned into a spirit and managed to escape from the hunters. His spirit flew to Ningenkai and managed to possess the body of an unborn child in a woman's womb. When the child was born, he was given the name Shuichi Minamino and was loved and cherished by his mother, Shiori. Shiori's husband died before Shuichi was born. His true purpose of staying in Ningenkai was to recover his strength for mostly 10 years. But 15 years had passed and still, he's still with Shiori. He can't bear to leave his human mother after all she had done for him. Shuichi stayed and took care of her especially in times when she needs him.

Shuichi unexpectedly turned back as a youko during the Ankoku Bujutsukai. This was when his youko side was introduced. This was made possible by Uraurashima's curse that should have made Shuichi a helpless child, but instead he turned into a youko: his original form. He went back to his human form after their match.
At the time when Yusuke died for the second time, Shuichi unconsciously transformed into his youko form by himself as a result of his vengeful spirit.
His third and final transformation was during his match with Sigure during the Makai tournament. But Kurama decided to turn into his human form to defeat Sigure for personal reasons.

Kuronue is Youko Kurama's late friend. He was introduced in the second movie of Yu Yu Hakusho. He was Kurama's companion everytime he steals in Makai. Kuronue died while he tried to retrieve his necklace that is very important to him. He jumped into a deep hole underground and he was attacked by razor sharp bamboos. Kurama could not do anything but to save himself , and so he left him. Do not take it the wrong way. He also mourned for the loss of his friend.