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Relationships
Genjyo Sanzo
Sha Gojyo 
Being the first person of the ikkou whom Hakkai met, he obviously has a much closer relationship with him than the rest of the ikkou. They're both best friends. 
Son Goku
Nothing that I know of, other than an older brother figure. *grin* 
Yaone 
It's not known what kind of relationship Hakkai has with the charming female member of the Kougaiji-tachi, but it's thought that he compares her to his dead older sister and lover, Kanna.