Seina doesn't exactly control probability, but his luck is so bad that the probability of anything occurring around him is likely to be changed, sometimes in a predictable way so that this trait can be used as a weapon. Others can control what might happen oleh placing him on the scene, if they're willing to risk with the collateral damage.
Fullbring is focused through a brass knuckle on his right hand. The knuckle has a pattern of three sevens in a row. oleh having anything come into contact with it, he is able to manipulate the probability of that and hit a jackpot.
