The strength of their bond was evident when 26-year-old Alex - the owner of trendy London gallery 20 Hoxton Square - joined Monaco's first family at a Monte Carlo showjumping event, where the striking royal was competing.
In between putting her horse Tintero through its paces, the 23-year-old found time to share intimate moments with her beau, who is the son of millionaire property developer Guy Dellal and his Brazilian model wife Andrea.
Exchanging lingering kisses and glances, the young lovers at times appeared wrapped up in a world of their own.
Alex, being very much part of the Monegasque clan now, also chatted with Charlotte's mother, uncle, Prince Albert, and sister Alexandra, Caroline's daughter with Prince Ernst of Hanover.
The little girl has been seen in Monte Carlo much more since the princess relocated from their marital home in Fontainbleau, France amid speculation about her marriage.
Completing the family group was Pierre, the younger of her two brothers. Now a handsome young man he attended with his girlfriend, Italian heiress and model Beatrice Borromeo, herself no slouch in the looks department.