The 'π' character is new. The lower-case version is also π' because unicode fonts do not yet provide a lower-case equivalent.' It represents a consonant speech sound used surprisingly often in the English language but that has never had it's own character. The official 2-letter digraph is 'zh' but that is seldom seen in actual word spellings.
This sound is the voiced version of /sh/. If you say the /sh/ sound and hum at the same time, you'll make the 'π' sound. Since 'X' represents the 'sh' sound, it makes sense to use an 'x' with a crossbar to represent the 'zh' / 'π' sound.