We are yet to know how the story takes place, but here's the basic rough draft.
Tidus is in some sort of afterlife. Yuna finds out there is a way she can bring him back, but the only person left is Rikku. Lulu got pregnant (maybe by Wakka?), I think Kimahru died or he went back to his home place, and Cid is back but just as transportation.
Rikku joins Yuna to bring Tidus back and she meets Lenne and the Nameless Person on the way. The nameless person and Lenne are rather close, and although Yuna and Lenne look similar, they're not related.
And that picture where Nameless Man is holding Yuna is probably because when she met him, she thought it was Tidus and reacted suddenly. These are all assumptions aside from the basic intro.
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