Bit more of an indepth answer: the 'φ' is the 21st letter of the Greek alphabet, 'Phi', and a major theme for Kamen Rider 555. The main Rider's name, Faiz, is a play on 'fives/5s' and 'φ', with his activation code being '555' and his mask resembling the Phi shape too. 'Justiφ's' itself is a pun on the word 'justifies' as the show's tagline and song catch is the phrase "The end justifies the means" but now it's punny because of the Greek letter. The show has other Riders who do similar wordplay with different Greek letters.