Joan of Arc is to me the epitome of a strong INFP on a quest. the side most people see 90% of the time is a sweet, creative, dissorganized, romantic and relaxed personality that enjoys intellectual conversation (despite being dominant feeling, many of us are extremely logical. INFJs are extremely intelligent, but frankly most of the FJs I've met are extremely illogical) and has an unusual connection to animals and small children (I am 19 and almost every time I meet a little kid, they think of me as their older brother in less than a day).
However, that all changes when our values are violated. A strong INFP standing up for his beliefs can talk with extreme amounts of authority and charisma and sometimes can sound much older than they really are. When this switch is flipped, the INFP's normally relaxed, carefree nature becomes merciless and vindicative. We become the shining hero who fights against much stronger oppononent but triumphs through sheer resolve. This sounds a ton like Joan of Arc. she was only a 17 year old girl but lead armies and took numbers of forts commanded by seasoned verterans.
Some of the confusion comes from the fact that INFPs are often more "J-ish" than INFJs. for example, the greatest strength of an INFP is one of judgement while the an INFJ's greatest strength is the powerful perception of introverted intuition. INFJs also tend to be much more merciful and pacifistic, INFPs have no mercy for anything they believe to be pure evil. also, with Joan of Arc, we wouldn't have heard a lot about what she was like when she was relaxed, so this could lead some people to assume she's J