Yeah it feels the same way to me, Karlach although is incredibly awesome doesn't feel as fleshy as other companions.
I romanced her from the start and have her in my party always, but she always feels like she is just a side-character tagging along, both in terms of adventure and romance. Lae'zel who I decided to co-romance just until Karlach is ready for it turned out to be such an in-depth relationship that it developed so much further that I was planning for it to go. Meanwhile Karlach is barely making any romantic moves even after giving her the 2nd iron.
And in terms of personal quests... Lae'zel and Shadowheart both have such massive quests which lead them to their own acts basically. Just look at how long Shadowheart's journal entry is... then look at Karlach who has only two puny lines (we defeated the fake paladins) and collected two Infernal Iron. Just sad really...
