Julien was the son of Marius Boudreaux, head of the New Orleans Assassins Guild and Bella Donna's brother. He all but officially ended Gambit and Bella's marriage by challenging Gambit to a duel on their wedding day. He didn't want peace between the Guilds and he didn't like Gambit with Bella. There have been hints that Julien had an unnatural or extremely overprotective feelings toward Bella Donna.
Gambit ended up killing Julien and was banished from the Thieves Guild and from New Orleans in order to keep the peace. Julien was brought back to life by Candra and her Elixir of Life. It made him even more insane than he was before and Candra used him against the Thieves and Gambit. He was killed again by his father and the Assassins.
[See the Ghost Rider/X-Men/Brood crossover and the 1st Gambit LS] (from Gambit Guild)
Once head of the New Orleans Assassins Guild, he sought peace with the Thieves Guild. Working with Jean Luc LeBeau, the two made a deal which arranged the marriage of their children, Remy and Bella Donna Boudreaux, who were already childhood sweethearts, in the hopes that this would end the fighting. The attempt at peace was unsuccessful and cost him his son, twice. He recently passed away, possibly under mysterious circumstances, and Bella Donna is now head of the Assassins Guild.
[See XM8, and the 1st Gambit Limited series.] (from Gambit Guild)