[quote=@Saltwater Thief]Well, it's incomplete due to still needing to come up with one last Weakness...[/quote]
You could try 'Bastard', making the nature of his birth a point he'd have to work against in court, where some of the other Councilmembers would have problems working with someone who had no claim to be amongst them other than their Queen's word.

Not that Carmella would have a problem with him. She's found most bastards to be far more trustworthy than her fellow nobles, who had far more secrets to hide in their extended familes than a man alone could ever have to worry about.

Another option would be to make his continued existance a threat to his family's honour, and given their suppression of his father's will they're willing to use underhanded tactics to keep him out of sight. Have they gone as far as making arrangement to have his life shortened, to make sure he doesn't cause problems for them in future? Or would this mirror the 'Black Sheep' trait he already has?

Other than that, maybe something about his temperament, such as 'Hot-Heated', 'Brutal' or 'Blunt'? How would Sir Martin approach a situation, and how would his attitude to life affect the policies he would would put forth or support?