An NPC ally of a character of mine has a debilitating disease (I have chosen MS).
What mechanical effect would this have on the character? I don't think it would be covered by the flaw mechanic as the NPC wouldn't get to decide when to give in to the flaw, and I think NPC don't need to recieve XP as incentive to roleplay. I'd like to represent it as the abilities of the character being unreliable (in the sense they could have good or bad days of MS).
I'm basing this unreliability on this quote from Wikipedia
MS takes several forms, with new symptoms occurring either in discrete attacks (relapsing forms) or slowly accumulating over time (progressive forms). Between attacks, symptoms may go away completely, but permanent neurological problems often occur, especially as the disease advances.
Emphasis mine. I'm mostly interested in the relapsing form.
However I don't know enough about this disease or the mechanics of nWoD to say how to represent an unreliable ability score.