Lol, yes, thats Nozomi's errm...how do you say it...token "problem". Like Kouta being afraid of relationships, Nyuu turning into Lucy, Nana losing her legs and arms, Bandou being a total c**k, Mariko being NUTS, Yuka liking her cousin, Mayu not being able to trust people all that well, all the Kazukawa men being INSANE in the membrane, that one girl not being able to take a shower...you know. Nozomi is shy and wears diapers. xD Its not THAT different. It goes along with the fact that everyone has problems /faults /insanity.
She was abused by her father (who I BELIEVE is Kazukawa, or else they look very similiar) and the trauma caused her to develop stress incontinence. So whenever she gets scared, very shy, flustered (like when Nyuu basically rapes her) she evacuates, or almost does. Kouta is the first, and probably ONLY person to know what Nozomi's "secret panties" are. He looks out for her as much as he does Nyuu, and Yuka. She first appears in the 5th volume of the manga.
And thats my Nozomi spiel. ^__^