I don't understand the end of okamiden. When kuni is walking away and talking..

That's after the fight with Dark Chibiterasu. It turns out Kuni was adopted by Kushi and Susano-o. Kuni wants to find out who his real parents are, he gets into an argument with Suasno-o and Kuni plush Chibi leave. Kushi calms Suasno-o down by telling him that she is expecting. While Chibi and Kuni are leaving the village, Kuni stops and tells Chibi he needs to do this alone. After Kuni leaves, Sakuya appears and gives Chibi her thanks for saving Nippon, which is just a kiss. Issun appears in a flying saucer (that was on the card he got in the ruins) and tell the two that Gen built it for him so he can go check on Ammy (Or Amaterasu). Chibi then hops in the UFO with Issun and they leave for the Celestial Plain.

It's not really clear who Kuni is. With Kushi saying she wants to name her child Kuni, it could be that Kuni really is the child of Susano and Kuni somehow displaced in time. He could be from the future. That could also explain why Nagi's armor suited him so well - because he would be part of the blood line.

Another theory, though it's particularly unfounded right now, is that Kuni is Nagi, displaced in time, but that idea is kind of killed by the fact that adult Nagi in the game seems to have no recollection of these adventures with Chibi when we meet him in the game. This could easily be explained away in the next game with all the craziness and loopholes that time travel comes preloaded with though.

Shure Kuni said 'Akuro 9hatever0 said he was a vessal of power so he desaided to leave to hind out his real name and his real self.sooo he left but susano said they were no more father and son so as he found the she was in birth they decieded if the baby was a boy they will name him Kuni instead.

The adult kuni is the narrator who speaks in the beginning of the game.

A theory of mine is that Kuni was somehow infused with the spirt of Nagi before Susano found him. This is supported by Akuro referring to Kuni as a "vessel that holds a great power". This would explain why Kuni was able to hurt Master Anura and use Nagi's armor. Also, Kuni is supposedly based off of Prince Ōkuninushi, a famous kami in Japanese mythology. However, Ōkuninushi was described as being Susano's son, and by the end of the game it's clear that Kuni is not. This leads to the possibility that "Kuni" is not the actual character whom Ōkuninushi is based on, but rather Susano's true son, whom Kushi is pregnant with and says she will name Kuni if it is a boy, is the true equivilant. While the stories of other characters stayed close to their original Shinto counterparts, Kuni's story hardly resembles the story of Prince Ōkuninushi at all. None of the events described in Ōkuninushi's tale ever occur in the game, although they may take place on the journey Kuni sets out on after the game, but that's somewhat unlikely as too many factors debunk it.