You're a fire elf, living most of your life on a volcano — Being in contact with fire became a mundane thing for you, you master it, a little bit of heat won't scare you away from her touch.

"Soldier, I can't believe it, are you sure, like, really sure it's fine?" Karlach asks still hesitantly, the tiefling can't help but worry, you're resistant but it's not like you're immune to fire. She doesn't want to hurt you in any form, she can be patient, she can wait for you if needed, no rushing.