First Appearance

Making his debut in Uncanny X-Men #305, Ambassador St. Croix was portrayed as an elderly, white man. He reveals that he is a he is a member of  the Mutant Underground  since its inception and hes friends with Professor X  and Moira MacTaggert . He is also revealed as a mutant-rights activist which made him the target of fellow politician Senator Robert Kelly  a well known mutant hater. He Later tells x-man Storm the reason for his activism on behalf of mutant kind. His wife, Lenore, died in a car accident because her mutant powers had turned half her body into a black crystal-like form. As a result the paramedics refused help, and he held her in his arms as she died. 
 

History

Early Life

Cartier grew up wealthy and pursued a career in politics. After meeting an unnamed wife they wed and she had a son Marius. Few years later they welcomed Monet his second child and she quickly grew into his favorite.  As him and his son's relationship deteriorated he grew closer to Monet/ M. Even after his wife had two more kids, Claudette and Nicole/ M Twins Monet remained his favored child. When one of the twins grew Autistic he spent all his money to get her the best help money could buy but in the end her condition remained the same. 
 
His world changed one night he took the girls out for a ride in his car. When he returned his wife was dead and his son was kicked out of the house because he had failed to protect her. Realizing his daughter Monet was a mutant he brought her to the attention of  Professor X with whom he secretly allied in the 
Mutant Underground. As time went on normal returned to their lives despite his wife's cause of death never found out. As he returned from a political rally he found only Monet with the twins mysteriously gone. 
 

Losing his family

With just Monet left he tightened security around her especially as no trace of the twins or even Marius could be found. Monet had also started having strange seizures so he always ensured she had a guard nearby. Monet was soon abducted by the Phalanx of Earth and after the battle he enrolled her in the Emma Frost  and Banshee's Xavier endorsed school. (Unknown to him the M he had been with since the twins disappearance was the twins in disguise).
 

Trying to make amends

He continued his life of politician and ambassador checking in routinely on Monet. When he heard that the twins and Monet were at the school he dropped everything and rushed to see them. Overjoyed to have his family back he was both shocked and happy to hear that Marius was there as well. Despite Monet telling him Marius was not there he visited a weakened Emplate with her. Here he was told by the villain that he had killed his mother years before and that "Marius" was dead. The bitterness of his son over his favoritism years before hit him hard.
 
Trying to do the best for his children he took the twins with him and enrolled Monet in a private school. Marius soon escaped. Soon however Monet convinced him the best place for her was with Generation x and he sent her back there. He continued his political career and it is assumed the twi ns are with him.
 

Re-Emergence

He reappeared after he had been taken prisoner by a terrorist who wanted his daughter Monet for her abundant energies.  Monet was informed of his capture and discovered he had been kidnapped by Baron Mordo, who was dying and needed Monet's life force in the same way that Emplate had. Mordo was tricked by Monet's mental abilities and her dad was safely rescued. 
 

Inconsistent Details

 
Reconciling his history
Combining both version of the "Ambasador" and since we know that Marius killed the mother of all his kids it is possible that she was  his second wife. 
 
Different people?
Cartier St. Croix and Louis St. Croix  are not two characters but one.  

Evidence-  Both are ambassadors and members of Xavier's Mutant Underground. It is taken that either Cartier or Louis is his middle name