Why does any character custom or another follow another.
Why would Shadowheart follow La'eazel?
Astorian follow Blade of frontiers?
Gale follow Shadowheart? (okay it's cause of her eyes.)
As far as any of them know they are all a bunch of nobodies with delusions of grander, as far as they know the other is just a cranky as Gith that may or may not really have a tadpole in their mind (after all gith have psionics), or maybe they are working with the Illithid for when they (the others) turn, they have someone they can trust right next to them. or take them to the Elder Brain.
It's not back stories, it's not quirks of voice, or having quests specific to you that makes people follow you. More often then not it's the fact that your there, your making decisions they don't want to, or you have more of a plan then they do, one they can agree to. Not everyone wants to go to the Creche, not everyone wants to use the druids, some want to use the cambion. Yet they are following Taz McBladerson (sorry stole the name) because Taz is doing something, he's trying, he's making decisions that the others aren't sure of so hell let him take the blame, and if it works out all the better. If not they can bitch about it, and they do.