Okay, let's just all remember that a bisexual/pansexual woman is not a lesbian nor is she straight (is, however, a queer character by definition) and not derail this thread into THAT discourse on top of everything.
Bury Your Gays only works when it's a token rare gay character who dies in a whole cast of het people, and this game has a whole other cast of similarly bi/pan characters, including women, who are alive and can be happy, so it does not apply. It is, however, very much possible to ride a woman with nothing hetero about it, I promise.
We can discuss faults in Karlach's quest and writing without reaching for additional reasons, all queer characters needn't be immortal.
All said sincerely as a lesbian myself, so don't jump at me. Let's just stay on track.
Every single character companion in the game is technically pan, or as I prefer to call it solely when it comes to game design "player-sexual." I just found Karlach to be especially butch-lesbian coded, or at the very least Pan-Butch coded. Much moreso than Shadowheart and Laezel. As a Queer Butch myself who has also struggled with being stuck in a cycle of abuse for ten years, Karlach's story felt incredibly real to me and the way it was handled was incredibly poor.
Either way the sexuality of the character doesn't really matter since there's really no excuse for them to have handled her character's story so badly. Especially since the option to save her still exists when you play as Karlach with the core character option in the character creator.