And since I haven't yet typed up my scribblings, I thought I'd do a quickie post of the characters secrets for GH. (Obviously there will be spoilers below.) They all have one (except maybe Yasuhara). Many have been revealed over time in the manga/anime series. So, without further ado...
Mai: is really an orphan, and has been since Jr. H.S. when her mother died. Her father died when she was very young. Both parents were completely without any living relatives, so Mai has no other relatives in the world.--revealed in the Urado story
Naru: is really Oliver (Noll) Davis of England and a prominent member of the English SPR. He came to Japan to search for his brother's body--revealed in Heiki
Dream!Naru: is really Eugene Davis and Noll's twin brother. He'd accidentally been struck by a car and then deliberately run over--revealed in Heiki
Lin: is actually Chinese. His core family is the last of his extended family to leave Hong Kong and move to England before the colony was to revert to China.--revealed in the Urado story
Masako: isn't so much a medium as she is a psychometrist. Following Oliver Davis' theory of two types of mediums, she fits the psychometric definition more.--revealed in Yasuhara's school case (? I think that's where this was revealed)
Ayako: on her own, she has little ability or strength. However, when there's a living tree around and she summons the elemental spirits inhabiting the tree, her ability to purify souls is greater than any of the other members.--revealed in the Yoshimi case
Bou-san: originally, he was able to see spirits. However, after hitting his head badly, he lost that ability.--revealed in Heiki
Also, his hair is naturally so light and was worse when he was younger--revealed in Akumu ga Sumu Ie (Agawa case)
John: an exorcist at 19 yrs of age even though becoming a catholic exorcist requires being a priest first. Bou-san and Yasuhara speculate that he hasn't reached that rank yet and so isn't a true exorcist.--speculation in Heiki.
However, in John's case, it is then later revealed (much to Bou-san's and Yasuhara's surprise) that he is in fact a priest. ^^; --revealed in the Agawa case