Kratos has shitloads of brothers since ZEUS is his FATHER,
Hercules is one of them but all sons of Zeus are his brothers.
But people are mistaken when they use the sons of Zeus as his brother(s). They are his brothers when he became a GOD, nothing more. You cant be a god and not be in the family tree. So I wouldnt even call them brothers. More like "brothers".
The brother people talk about is his REAL brother from when he was a child =
Kratos was not an only child, and had, in fact, a brother. However, when they were young, while Kratos was deemed fit and worthy, his brother was deemed weak, sent to the mountains to fend for himself, where he died. Kratos lived on to become legendary, but his brother, coming of age in the Underworld, only had one desire; revenge against the brother that had abandoned him.
Little is known about Kratos' brother as of this moment, although from what it seems, his time in the underworld has made him something demonic in appearance. Although making no actual appearance in God Of War 3, it is heard in one scene where Kratos is referenced as the lesser of two, the exact phrase being "...Perhaps you should have chosen the OTHER one!", which foreshadows the possibility that Kratos's brother may appear later in the series, in ADDITION to the post-credits ending of God Of War III that leaves Kratos's fate ambiguous.
Another interesting thing =
"The forgotten warrior skin" I have it on GoW3 (got the key when I pre-orderd yay). The forgotten warrior might just be his REAL brother. The description fits him aswell. Watch this video from GoW 1 = http://www.youtube.com/watch?v=HdIOCVp36uQ
