6090 carries a special meaning in Greatest Love. Because of Dokko Jin’s heart condition (See: Heart), Dokko Jin must keep his heartrate in the 60-90 beats per minute zone; it is his safety zone. He even uses his Mind Control Time to make sure his heartbeat stays regulated (See: Mind Control Time).
After his first encounter with Gu Ae Jung, Dokko Jin’s heartrate begins to extend out of the safety zone every time he hears the Thump-thump song by KBSN. It continues to persist due to their frequent interaction, causing him to think that there is perhaps something wrong with his heart again. He eventually learns that he is attracted to Gu Ae Jung and she disarms his heart every time he is near her or thinking about her.
6090 becomes such an important symbol for Dokko’s safety zone that he even uses it as the code for his home’s alarm system. When Gu Ae Jung tries to help Jae Seok out by offering to go to Dokko’s home to deliver some water, she catches him exiting the shower. Embarrassed, she tries to hide thinking she’d be able to sneak out unnoticed. Things go awry when her phone starts to ring causing Dokko’s heart to go Thump-thump. Freaked out, he realizes that he’s not alone and sets up the alarm. Gu Ae Jung inadvertently sets the alarm off and has to explain her presence as well as some other misconceptions (See: Panty/Medicinal Tonic). Unable to leave since the house is on lockdown, Gu Ae Jung argues with him. Dokko casually relays that because of her his life has been turned upside down causing his heart to periodically extend outside of the 6090 safety zone. This clues Gu Ae Jung to his secret code and she uses it to exit, while a stupefied Dokko Jin looks on.
Later on, the 6090 number can no longer be guarded since Dokko’s actual heart becomes damaged causing him to get it fixed through another surgery. Without his safety zone, Dokko Jin needs to rely on recharging (See: 충전 (Chungjeon) – Recharge/Charge) with Gu Ae Jung instead.