[@Sosuke]Alright first off I'm a bit confused. Is this person someone else like the benkei situation, or Muramasa. I see you're trying to fit the group together using the third skill. Point being it has to be one singular person. I'll use Fake Assassin as an example. Kojiro is just Kojiro. Whatever rice farming peasant he was, or rogue ronin he was prior to be lifting into the position of the fictional Kojiro Sasaki is no longer relevant he is in all forms just Sasaki. So with that being said they have to be either Muramasa, or whoever. The grail doesn't stick people together with the unusual inclusion of Thousand Personas Hassan (who are all technically one person split into many), and Bonne/Mary (in which case its two seperate people as one servant and the both of them die in case of a single fatality).  

Second the ability to cut through magical armors, and such is fine however cleaving through B Rank noble phantasms is a bit much so I'll have to ask that you not do that. It basically means that anything below B+ is at danger of being ruined by simply combating your berserker.