Teldryn left and then he (the adventurer) died.
Oh, than let's look at Serana: I'm a vampire! And I carry a powerful object, and I was locked away because my daddy became obsessed with something, and he stopped caring about me and my mommy, then my mommy began to "battle" with my daddy and they locked me until a brave hero came and rescued me, and we fought my bad daddy to save the world.
Personally I have mixed feelings about Serana and that comes from how I approached her in several playthrough. My main took her as a follower just like any other, my vampire RP thought of her as a sister, and my Paladin thought she was an annoying companion that would help him claim Ariel's Bow. So I've had her fill different roles for each of my characters, but I'm still unsure how to take her myself.
I find her "emo vampire" look kinda beautiful (I'm a sucker for that sorta look) but also find her constant talk about certain areas annoying. After hearing her say something about a cave as I passed three in a row I just wanted her gone. Overall I think she can be helpful and pretty but also be a huge annoyance similar to all followers in the game with their either constant talk or terrible lines.
I don't use companions, only when they are forced via quests. From these Serana really stood out - with all her complaining she was hundred times better than Farkas or Aela That said, I find her complaining sad, not annoying. She loves the sun and daylight but due to her vampirism can't enjoy them fully - thus constant change of feelings