Can some one tell me whether Jason & Neri fall in love, and if so, in which series, on Ocean Girl...and do they have a relationship between them?
Yes, and in Series 4
To be specific, in the last episode, S04E26, Neri and Jason share, according to the wiki, a "kiss of true love."
Here they are, about to kiss.
Yes, I would say that Neri fell in love with Jason.