It is most definitely related to their personality and fighting style, why they`re all there. From what I know of the series involved, they're all similar either in persnality or fighting type.
Tsukuyomi Mana MuvLuvMitsurugi Meiya MuvLuvRemilia Scarlet TouhouLian Mao Mobile Suit Gundam Battlefield Record UC 0081Hong Meiling TouhouDomon Kasshu G GundamTakemikazuchi Type-00F Takamura Yui CustomRider Fate/stay nightMaster Asia G GundamSaber Fate/stay nightSion Eltnam Atlasia Melty BloodMaster Gundam G GundamKawakami Momoyo Maji de Watashi ni Koi ShinasaiGespenst Mk. II Super Robot WarsTakemikazuchi Type-00RTakamura Yui MuvLuv Total EclipseLamia Loveless Super Robot Wars