A thing to remember here is that these posts are strictly meant for the particular planet position. Like, today we are looking at Ketu in Pisces, so the description given here may change for you if your Ketu is although in the sign of Pisces but in conjunction with or aspected by other planets. -

Just like Rahu, Ketu also represents illusion as both are illusionary planets but the difference is that Ketu is also the one who removes the illusion and brings enlightenment. Besides this, Ketu represents isolation, separation, spirituality,mystical world, other dimension means things related to alien world, also represents Son, Dog, Intuition, research ability, curious nature, things which we have already achieved in past life and not much concerned to perform in this life.

As Pisces is the 12th sign of zodiac belt, it represents the same energy and things which are represented by 12th house such as Isolated Places, Spirituality, Imagination, Other Dimension or Other World etc. Besides these things, it also represents your Hidden Talents, Ability to work in background, Spiritual Self etc. Pisces is consisted of last 2 and half nakshatra, i.e. Purva-Bhadrapada, Uttara-Bhadrapada & Revati. Pisces' lord is Jupiter.

Recently I have come across of the view that Rahu and Ketu don't have aspects on 5th or 9th house, for the simple reason that they are shadow planets and for aspects you need a sight not shadow. I am yet to verify it on at least 50 charts, so for now I will continue with traditional approach of 5th and 9th aspects of Ketu.

As much as it looks like an ideal position for Ketu because of Spiritual Ketu is in Spiritual Pisces, but unfortunately it is not. Ketu does its job of isolating/separating everywhere and Pisces is no exception. Wherever Ketu is, person never finds completeness and wholesomeness in that area of life. So, Ketu in Pisces means person is dissatisfied with his spiritual progress. He is unable to find the true spiritual knowledge from his Gurus. He always feels there is something more to know in the name of eternal truth. As Pisces is also sign of intuition, imagination and Creative Thinking, they will find similar emptiness in these virtues too, If Jupiter and Venus aspect or conjunct Ketu in Pisces, it can make a person reach heights in Spirituality as both these planets are planets of faith and devotion. They are seekers rather than believers. 

As always, Ketu's placement in different nakshatra will change this scenario as per the position of Nakshatra Lord. If Ketu is in Purva-Bhadrapada Nakshatra, then Jupiter will have its impact on Rahu. If Ketu is in Uttara-Bhadrapada Nakshatra, then Ketu is impacted by Saturn. Likewise, if Ketu is in last nakshatra of Revati, then its lord Mercury's position would impact Ketu.

Sign lord wise, Jupiter is lord of Pisces. So,Jupiter's position will have highest say on how Ketu works in Pisces.