It's not that complicated, mari is the oldest and maru was a second child in her previous relationship who passed away as a baby. After that the parents separated the dad took mari with him and basically cut of the contact with mom. Now after a while gongja was adopted and since the name was on him when he was found nobody changed it.
thanks for the explanation, though the gongja name was just a bit lol
Uhh Mari was mentioned in earlier chapters. I think you need to read the chapters properly rather than skimming it.
its not that deep if i dont remember all the details
what is the family tree exactly?? i thought there is only Maru, but there is Mari?
with the naming pattern im surprised gongja is gongja, not maro