Compared to Fire Emblem Awakening, as well as Fire Emblem Fates, Fire Emblem Three Houses is rather wise. Romances remain quite secondary, there is no doubtful scene with the possibility of caressing one's allies, and above all, there is no magic offspring that reach adulthood instantly or via a time travel. However, there are several dozen special dialogues between the different characters as they develop their relationships, sometimes friendly, sometimes romantic, including with your protagonist, Byleth. We will avoid revealing elements of the scenario, while listing the important elements for developing relationships between characters.
Impact of gender and Byleth's choices
You will not be surprised to learn that the gender of your avatar will have an impact on the romances that will be open to him. There doesn't seem to be a strictly homosexual character, so if you have a character in mind with whom you want to develop a relationship, the easiest way is to choose the opposite sex. That being said, some characters are bisexual, and you can freely start a romance with them no matter what genre you choose. It should be noted that the female version of Byleth has many more options with Dorothea, Mercedes and others, it seems that very few men are open to it in FE3H, only Linhardt to be exact.
What will really have an impact will be the choice of your house, and then some of the branches in the story. The leader of each house, Edelgard, Dimitri and Claude as well as their right-hand man, Hubert and Dedue, are options only in the road to their house. A detailed list of all possible romances by route will be added later. Some characters are not met on certain roads, and others become irreconcilable enemies. Flayn, Seteth, Rhea, Gilbert, Cyril and Catherine are not available in some branches of the Jet Eagle Route among others, Jeritza and Jeralt are never options. Students from other classes, as well as academy staff, other than those listed above, are possible choices in all routes, as long as you recruit them before Chapter 12.
It is only possible to have S-level support with Rhea on the Jet Eagle route, by making a certain choice.
Written by Robin "Raiden" Bouquet. Translated from French by Millenium.us.org