Adair is the twelve-year old son of Fabian, Earl of Titan, whom Elodie meets in week 17 at the Grand Ball, and Cayleigh, Duchess of Elath, who died in childbirth. That makes little Adair the future Duke of Elath, an important territory whose rulers have suffered a number of misfortunes in recent years. Cayleigh's mother and sister are suspected to have been victims of agents from Sedna. Adair is the last heir of the bloodline remaining.
After Fabian's death (week 20), Elodie must choose who will be regent of Titan until Adair comes of age. The options include his stepmother; Arisse, his stepsister; Adele, his grandfather (Cayleigh's father Erwin), Elodie's uncle Armand, and Elodie herself through marriage.
Adair does not seem adept at dealing with girls near his own age, from his reactions at the Grand Ball. If Elodie chooses to marry him, he will be brought to live in the palace and there will be a later mention of him where Joslyn asks Elodie how he is faring and Elodie describes an interaction in which she saw her young fiancé catching frogs. You also get the option to speak with him on the weekend, during which he will ask to kiss you, seemingly thinking that he's "supposed" to.
If they do get married, their epilogue states that their relationship was closer to that of good friends and siblings, rather than 'passionate lovers'.