To the best of my knowledge, nobody knows the ending of the story but the author herself. There have been six LN volumes to date, and the sixth one does not appear to be the last one. In addition, there have been two spinoff volumes. See
this page for a complete list.
A summary of the series by volume:
Spoiler:
(1) Introduce everybody. Mitsuo dumps Kouko, Banri confesses to Kouko, Banri discovers that Linda meant something to him before he lost his memories.
(2) Banri and Kouko get to know each other better. Banri, from subconscious memories, rejects Kouko because she asks him to wait. He rejects Linda as well. Kouko pursues him, catches him, and confesses to him.
(3) Banri and Kouko start dating. Banri decides to throw a masquerade party to revive Mitsuo's spirits.
(4) Banri's memories come back briefly, and then are lost again. In the process he gets pretty banged up. Everybody (Kouko, Linda, Mitsuo, 2-D, Chinami(?)) takes turns nursing him, but Kouko does it over the top (as you might expect). They begin preparations to go to the ocean during summer vacation.
(5) All or most of them go to the ocean. Somehow, Kouko and Chinami wind up buddies. Lots of action with all of them in swimsuits, some of it scandalous.
(6) Kouko blames herself for something that happened, and temporarily goes hikikomori. During this time, Linda invites Banri to a fireworks party with the Omaken.
Exactly where NANA fits into all this, I do not yet know, but I do know that she and Banri become friends to some extent.