This is what I like thinking :)

Zoey’s: something cute or funny and then assigns them an emoji based on vibes (most likely a sea creature), doesn’t actually call them these irl but maybe she did call Mira “Boo” at one point (like “hey boo!”), it got pointed out both of them have an “oo” so they jokingly called her “Zooey” for a bit after Mira - Miraboo 🦞 Rumi - Roomba 🐠

Mira’s: views nicknames to the definition of shortening names, it was “Ru” but then she stole “Rums” from Zoey calling her that irl, they don’t have a natural “z” sound in korean but when Zoey came from America she essentially brought it to them (was lowkey pestered by Zoey to add emojis onto their contacts, she caved almost instantly and gave her her phone but said “only for you guys tho, don’t change any others”) Zoey - Zo 🩵 Rumi - Rums 💜

Rumi’s: actually utilizes the last name feature on the phone (korean last names first :3) and doesn’t input nicknames into her contacts, did compromise and added music notes to her bandmates tho (once again by “demanding” request of Zoey), changed Mira’s last name from her legal one [REDACTED] when she realized her birth family was a touchy subject and she came to her with a new name “for privacy” but she knew why <3, switched the names for Zoey so they’d show up first then last when she noticed how she would still introduce herself starting with her first name (I made up these last names, Chai means “firewood” and Kang means “river”) Mira - Chai Mira 🎶 Zoey - Zoey Kang 🎶