Overview
This is based on my experience. Feel free to correct me if you find any error.
Raising a ship’s affinity
Ships have affinity level that can be increased, starting from “Stranger”, then turning to “Friend”, “Crush”, “Love”, and finally, once you’ve given them a ring, “Oath”.
From what I’ve observed, simply using the same ship over and over again won’t raise the affinity to the max. Each stage of affinity can be achieved by doing specific actions. Those actions include:
1. Using the ship in 10 battles
2. Setting the ship as secretary and do 10 battles. Using the ship is optional*
3. Raising the ship’s level to 100, then do Cognitive Awakening**
By doing these actions, the ship’s affinity will increase by one stage until “Love”.
*= Point 1 and 2 can be stacked, meaning if you set a ship as a secretary when she still has “Stranger” affinity and go into battle 10 times with her, the affinity will jump to “Crush”.
**= Cognitive Awakening can be done by raising your ship to level 100, collecting their M Isomer by fighting them in the story mode, and exchanging them for Cognitive Chips in Akashi’s store. Once you have, simply go to Dock and open the ship’s status screen. Press Triangle (PS4), Y (XBOX), or I (Keyboard) to proceed. (Need to finish story mode first, up to chapter 6 and S-rank all the stages)
Buying the Promise Ring
Once you’ve raised a ship’s affinity to “Love”, it’s time to buy a Promise Ring for them. The rings are available for purchase once the ship’s affinity became “Love”.
To buy the ring, you need Heart Tokens. Heart Tokens are unique to each ship, meaning you can’t buy Shimakaze’s ring with Suruga’s Heart Tokens. Heart Tokens can be acquired through two methods:
1. Watching an episode for the first time in Episodes mode. You can only get Heart Tokens once from each episode.
2. Clearing Extreme Battles. You can get a ship’s Heart Tokens by fighting them. For example, clearing stage 1 will get you Shimakaze’s and Yukikaze’s Heart Tokens.
Once you’ve collected 25 of Heart Tokens, you can exchange them for Promise Rings from Akashi’s store.
Marriage
With the ring in your possession, simply go to the Dock and open the ship’s status screen. Press Triangle (PS4), Y (XBOX), or I (Keyboard) to start the marriage.
And you’re done! The affinity on your ship will now show “Oath”. I haven’t checked it, but one of the tutorial said by doing this, your ship’s status will be boosted by a certain amount. Enjoy!