next thing you know hideo kojima takes over and lilith actually works as a secret agent for a set of individuals that have controlled the world governments ever since the creation of the baking system called solus.
Yuuri's father used to be part of them but due to him growing sick of the organisation thus hiding away with the genetic code for the creation of a bioweapon, which he could not destroy due to it being the only reminder he has of yuuri's mother.
Thus born in the battlefield during WW2 lilith an expert trained in assassinations by solus, gets assined in a mission to become the agent that retrieves the genetic code.
However lilith falls in love with yuuri and delays the wrath of solus as much as possible in order to protect her new family.
...
Somewhere in here add that yuuri is actually the finished result of the bioweapon based on dna from the head of solus a legendary general called big boy.