Mash Kyrielight

In her base and Ortinax forms, Mashu is the only Servant to have 0 cost, however when she's in her Paladin form, just like all SSRs she has a cost of 16. She is also the only Servant so far to have her rating changed, starting out as a 3★, getting promoted to 4★ after Chapter 15, Arrow 1 of Camelot, and reaching 5★ at a certain point in Trinity Metatronius.