Louis of France born in 1244, was the firstborn of the famous Saint Louis IX of France, he died in otl in 1260, and was succeeded as heir and later to the throne by his brother Philip, who became Philip III.
Now, Louis was betrothed to Berengaria of Castile who at the time was heir to the throne of Castile, before being displaced by her brother Ferdinand's birth. If Louis does not die in 1260, and instead lives on into adulthood, I suppose the chances of his marrying Berengaria are quite high.
This leads me to another query, if he is alive and is heir to the throne of France, who does his brother Philip marry? Does Philip marry Isabella of Aragon or a French nobleman's daughter, or someone closer to the French borders?
And, would Louis follow his father on the disastorous eighth crusade?