The 12v battery is a lithium battery attached to the exterior of the traction battery under the seat. Weighs less than a lead acid battery and from what I read, seems less problematic than how the lead acid battery that the PHEV and BEV models use. It is low capacity and has a relay that will open should there be excessive discharge when the car is not "running" to protect the battery and ensure it has enough voltage/capacity to power up the car when needed. There is a reset button on the dash to the left of the steering wheel that will close the relay when you are ready to drive. The need will be obvious as nothing will work - you will have to use the mechanical key to get into the car.
I've had to do this twice in the first month of ownership, both times related to an OBD device I left plugged in (never in the year since then). A number of the 2017 models had a defect where there was excessive drain on the battery and those owners frequently had to use the reset button to start their car until the defect was fixed. The salesman who sold me my car (I had an early 2018 model) pointed out the reset button, in fact the only feature he identified! I already knew about it from reading the forums, and had bought my car on the phone so he didn't need to "sell" the car to me.
The 12v battery is part of the "drivetrain" and so is covered by the 100,000 mile warranty. That beats the lead acid battery too.
2018 Kia LX HEV Metal Stream with Advanced Tech