Her previous master (more of those typical 'aloof hero who confess to dungeon boss/monster girl cuz love at first sight' type), also a Japanese, was the first to call her Ha(8)ku(9) instead of 'Core 'hachikyuu(89) because the 'core no 89' is a pain to say all the time.
I think it's when Rokuko introduce herself as Rokuko instead of Core no 695 that Haku's interest was piqued, then after wishing she has similar 'nickname based on her core number', Kehma mentions about the 8-9 reading