I think this is the default ending, getting them together but correct me if I am wrong.
There are a few Dialogue options scattered throughout the story to ensure them getting together.
When everyone is playing Truth or Slap, Make sure to steer away from Ren having to talk about Nona. He will thank you, if you were Successful
After the beach you head to the cave with Ren and Jonas. The conversation will lead towards Nona Pick the Option that says “ About Nona” and then pick the Option to tell Ren to “Go for it”
[Spoilers]
*** Spoiler - click to reveal ***When You, Jonas, Ren ,and Nona are on the Comm Tower and are deciding who should go with you to get the Boat keys. Take Nona with you
( I don't know if leaving her back will help with their relationship but it's what I did, and I still got the Achievement)
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Make sure to tell her that Ren would appreciate what she had to say about him, and that He likes her too.
If everything works out, Ren and Nona should split off to wait at the Bunker While you and Jonas go to open it. And once you return they will be together.
Hope this at least somewhat helps.