"Still the iron claw in the velvet glove, Mariel?"
- – Timov, 2259
Mariel was one of Londo Mollari's four wives.
Courtships and Intrigue
Before her marriage to Londo, a very young Mariel had been courted by two promising noblemen in the military; Solla and Riva. Solla was the more promising and at her father's instruction Mariel willingly accepted his advances, much to Riva's consternation. Not willing to let the matter drop Riva attempted to fight Solla for Mariel's hand though was soundly beaten. After which Riva publicly swore vengeance against his rival suitor and when Solla and his younger brother Durla both fell ill from poisoning, suspicion instantly fell on Riva. Though Durla survived, Solla eventually succumbed and and when the other members of their squad moved to arrest him, he very stupidly made to terminal error of resisting arrest.
With both of Mariel's suitors dead, the late Solla's younger brother Durla approached Mariel and confessed his love for her. Mariel had only been interested in Solla and Riva because they had potential and were destined for greater things had no interest in Solla's lesser brother, the "pathetic" Durla and cruelly dismissed the young man. Unknown to all at the time it had actually been Durla who had poisoned both his brother and himself and allowed the blame to fall of Riva.
Shortly thereafter, Mariel had become linked with House Mollari and her father arranged for her to wed the much older nobleman, Londo Mollari. Unnoticed by either newly wed, Durla had attended the bonding ceremony.
Married to Mollari
Mariel was the youngest of Londo's three wives by many years. Soft spoken and beautiful, she lavished affection on Londo, but did not truly love him. She maintained numerous affairs, as Londo explained she was attracted to men of power like a moth to the flame--but she burned them.
In 2259, Emperor Turhan offered to grant Londo any one wish that he desired. He requested a divorce from the arranged marriages to his three wives: Mariel, Timov, and Daggair. One condition was presented which was to keep at least one of the wives. He invited all three of them to come to Babylon 5 so he could deliver the news personally, just before having an anniversary party for his Day of Ascension. Whereas Timov made no pretenses regarding her feeling toward Londo, Mariel did what she could to ensure she would be the one he kept.
Not willing to chance him choosing someone else, Mariel bought an old Centauri artifact from a trader in the Zocalo. Mariel realized it had been booby trapped, and she gave it to her husband at the party. The trap poisoned him, bringing him close to death. Mariel pouted and protested her innocence, though the other wives paid no attention to her.
However, Londo does recover. While he recuperates in Medlab, Mariel visits one of her lovers--none other than Londo's hated rival, G'Kar of the Narn Regime. G'Kar explains that he knows full well she not only knew the device was trapped but identified herself as the giver to throw suspicion elsewhere. He warns her Londo is likely to figure it out as well, though Mariel does not seem concerned.
Ultimately, Londo chooses to keep Timov because he always knew where he stood with her. He does offer Mariel a modest alimony as a settlement, but she will no longer be able to live the privileged life of a noble she is accustomed to. 
- According to ("Soul Mates") writter Peter David; Mariel corresponds to "Death."
- During the scene in Londo's "Day of Ascension" party wherein G'Kar is being accosted by Daggair for wearing shoes (a grave insult, it seems), G'Kar is seen throwing a small object (a grape or something similarly sized) at Mariel, secretly. It has never been established canonically what that object was.