Pe is the seventeenth letter of the Syriac abjad, with the numeral value eighty. It writes p and is one of the six letters with hard and soft readings; soft pe is f. It joins on both sides. Its name is the inherited Northwest Semitic word for a mouth.
A reversed form of pe is recorded as a sign of its own in the modern character standard. It is used not in Syriac proper but in Christian Palestinian Aramaic, a separate Aramaic dialect whose speakers took the Syriac letters over for their own use and needed a sign the alphabet did not supply. That borrowing is one of several. Sogdian and Persian writers each added marked letters to the same set for sounds Syriac does not have, and Garshuni added marked letters for Arabic.
The pattern is consistent and worth naming: when a language borrowed the Syriac alphabet, it extended the letters rather than renumbering the alphabet. Syriac stayed at twenty-two letters throughout, and everything added stayed visibly additional.
Pe holds the seventeenth place in an order Syriac inherited unchanged from the older Aramaic alphabet, so the place, the numeral value and the name all come from the same inheritance, and the same three facts hold for the cognate letters in Hebrew and in Arabic at the same place.