Revealed in the pre-released scenes, after uniting the four clans, Fergus won Elinor's hand in marriage in a competition against the other lords, much like the lord's sons tried to win Merida. They were charged in a contest of swordsmanship, to which Fergus won and was married to the noblewoman, making her his Queen. She bore him their daughter Merida and later their three sons.
The exact relations between the two early on in their marriage is unknown, but it is incredibly clear that the two fell in love and act as such towards one another. Elinor's graceful and domineering personality offsets Fergus' abrassive warrior behaviour perfectly, commanding great respect from both him and the other lords to the point where they greatly fear disappointing her. She seems to have a natural tolerance for her husband's zany antics, ignoring the utter destruction of their dinner table and allowing the men to party and to go back to "avenging (his) leg."
When Elinor is upset, Fergus can pick up on it right away and manages to help her vent her frustrations (all be it with a comedic spin). When he believes Elinor to be dead, he immediately goes into a panic, taking the entire castle staff and all of its guests to kill the bear he believes to have killed her (not knowing that the bear was Elinor herself).
Note: Please add only ones that are part of the RotBTD fandom