How would Jupiter Pisces in the 8th/Mars Aries in the 9th house manifest for a person?

These factors depend on ALL of the 40-50 factors in the birthchart, since everything  there affects everything else. And the FIRST thing we look at is how a planet interacts with other planets ... the "aspects" ... we do this before we even consider the Sign it is in.

But understand that our chart does not control us ... it only indicates our tendencies, and no matter what is in our chart, we can outgrow it if we work at it.Jupiter is our need to network with others.  Well-aspected, we easily give and forgive. Badly-aspected, and we trust to much to luck and tend to not keep our promises, disappointing others, and being disappinted by others.In Pisces, our subconscious programs tend to rule how we use our need to network.  Badly aspected, Jupiter in Pisces deceives themselves when Jupiter-actions don't work out well, or becomes a martyr giving to others without getting back.  Well aspected, Jupiter in Pisces is capable of empathy and compassion and finds it easy to network with others in constructive ways.In the 8th house, our networking involves what we want from our spouse, and what our spouse gives to us ... and how much we learn from marital conflict and grow .. vs. how much we just argue.Mars is how we respond to anything we want ... so it is our ability to take action, how we take action, how we respond to frustration .. so it involves how assertive we are, how aggressive we are, how we handle anger.  Obviously, a well-aspected Mars handles their desires in skillful and constructive ways, while a badly-aspected Mars tends to be a bull in a china shop and a bit of a disaster.

In Aries, the desires will be urgent, the reaction will be quick and perhaps impulsive. Certainly the person is not afraid of taking action or asserting themselves, unless Mars is in hard aspect with Saturn, or some other planetary aspect indicates that they have bad experiences when trying to assert themselves and so they learn to be afraid.  Or more aggressive instead.

In the 9th house, most of their desires revolve around 9th house areas:society, legal matters, abstract thinking and higher education, religion.  I had a client with Mars in Aries in the 9th house who was devoted to their religion and also had spent their lifetime working for causes they believed in (working to establish labor unions in the 1930's, working to end the Vietnam War in the 1960's, etc).  Their action was directed towards changing laws and societal values ... both of which are 9th house matters. But ANY action that involves any of the 9th house matters could be how Mars in 9th expresses itself.  I have another client with Mars in the 8th, but in Virgo.  They.are not an activist, but they often do random acts of kindness for strangers .. a very Virgo way to take action for society (Virgo serves).