I think she is just kinda the default if you have no imagination - softspoken human female that isn’t totally obnoxious.
In terms of female romance options Lae’zel is the only one who it seemed to make sense would be forward enough to try and jump your bones after just 2 or 3 days. Karlach wasn’t even an option for me, she had no romance adjacent dialogue at the tiefling party. I wonder if part of that was just how late she joined up in act 1, I did get her engine “fixed” for the first time but it was one of the last things I did and I can’t remember if it was before or after clearing the goblin camp.
I did really enjoy Shadowheart’s arc though, especially since I generally adopted a policy to not suggest or try to interfere with the companions’ quests. Just left them to their own decisions without trying to sway them, and the fact that Shadowheart chooses to reject Shar and Lae’zel chooses to reject Vlaakith (or at least hear Voss out) made for two of the most powerful moments in act 2 IMO. There is lots to like about all three of them throughout their arcs and that’s part of what makes them so great.
Fun fact about the companion quests, what descisions you make as the player will change what your companions do if you let them make their own descisions. For example, if you side with the goblins and destroy the grove then Shadowheart will actually not reject Shar if you let her make her own decision.
Hm, are you sure? In my Durge playthrough I ignored the grove and she rejected Shar anyway, without me suggesting to do so. Thought for sure she was gonna skewer the aasimar.
I actually got surprised myself on another playthrough just yesterday where I was doing pure evil, but she decided to spare the nightsong anyway, which didn’t happen last time I did an evil playthrough. Maybe it’s random in some way, or maybe there’s a dialogue you need to have during some point that I missed this playthrough. I do know if you destroy the grove she becomes clearly upset about it during the party in camp, but I neglected to speak to Shadowheart almost entirely this playthrough so it’s possible I missed some dialogue trigger. I’m not sure myself now
I’ve grown so used to the companions falling in love with within a week in every RPG that I don’t even notice anymore. Personally, I legitimately like Shadowheart. I’ve tried the Karlach romance, and she wasn’t doing it for me. Dunno why. Lae’zel has more going on than I gave her credit for at the start, but Shart is still my girl.