Reaching S-Rank support in Fire Emblem Engage requires a special item called a Pact Ring. This ring can only be acquired late in Engage after completing a paralogue mission called the Connector. Paralogues are optional missions you can take part in that yield special rewards.
To obtain the Pact Ring, you must complete the previous Fire Emblem games and then complete the Connector paralogue mission. After completing the mission, you will receive the Pact Ring.
Once you have the Pact Ring, you can use it to increase your support levels with characters in Fire Emblem Engage. To do this, you will need to have your characters engage in combat and dodge and crit rate increases. You can also increase their support levels by engaging in romance options and giving them gifts from the Momento Drawer and Ally Notebook.
You can also increase your support levels by engaging in support conversations with characters in Fire Emblem Engage. Doing so will increase the relationship rating between the two characters and will eventually lead to S-Rank support.
Reaching S-Rank support in Fire Emblem Engage is a challenging but rewarding endeavor. With the right strategy and a bit of luck, it’s possible to get your characters to S-Rank relationships. Just remember to keep an eye out for the Pact Ring and engage in support conversations with your characters to increase the relationship rating. Good luck!