Recruit Karlach as early as possible
Keep Karlach in your active party often
Raise her approval by being kind, direct, heroic, and supportive
Help tieflings and other innocent people when possible
Avoid cruel, selfish, or overly manipulative choices around her
Complete her personal questline
Fix her infernal engine with Dammon
Meet Dammon in Act 1 and give him infernal iron
Meet Dammon again in Act 2 and continue upgrading her engine
Talk to Karlach frequently at camp
Choose flirtatious dialogue options when available
Support her emotionally about her engine and her past
Do not romance other companions if you want to stay focused on Karlach
Trigger her romance scene in Act 1 if your approval is high enough
Pick the dialogue that shows interest when she becomes available
Continue choosing affectionate and loyal dialogue in Act 2
Complete her Act 2 romance progression before moving too far ahead
In Act 3, keep supporting her and finish her story content
Choose the ending path that aligns with her survival if you want the best outcome for her romance