Basic Cailean-NPC reactions :
Aerie -- Aerie will be kind of nervous at first with his bad rep. He'd have to work on her a bit and earn her trust. Cailean feels sympathetic towards her and what she's been through. Pre-Cailean quest completion Aerie would tell CharName about how nervous he makes her. ("Even in the circus I heard such horrid things about him.") Post quest they have a talk about how good things can come from bad things.
Anomen -- Cailean finds him arrogant and uptight. Anomen finds Cailean lacking in restraint and somewhat gullible: how could a man be so foolish as to be taken in as Cailean was? Cailean does respect Anomen's profession even though he may not care for him personally (after all, Cailean's mother is a priest). Anomen could taunt Cailean about getting deceived by Masalina. Cailean could respond truthfully ("I suppose I was just another sucker for a pretty face"/"I suppose I really just wanted to be in love... Masalina made herself convenient and convincing") or snarkily (I suppose YOU cast 'detect evil' on every girl you meet").
Cernd – Finds his nature allusions a tad heavy-handed but will be somewhat respectful on how he deals so calmly with being the victim of a witch-hunt but incredulous on how he can leave his wife. Cernd will see him as young and impulsive. Perhaps they could talk about duty and responsibility towards loved ones vs the world.
Edwin -- Edwin is contemptuous of Cailean and Cailean is amused. Would also be interested in turning him in for a reward (more useful as coin than as adventurer).
Haer'Dalis – Cailean thinks he's out of his mind but hides it under a veneer of polite disbelief. Haer'Dalis is enchanted by Cailean's story; thinks it would make a great play. Late banter... Cailean's father, always wanting to be a bard, scandalized his family by expressing jealousy that some races were able to freely choose their paths in life, unfettered by archaic rules, and that it was no great surprise to them when he left his people to take up with a human.
Imoen – Sees a kindred spirit in her. Both enjoy a good joke. Maybe she sees him as brother-type figure? (minus the Bhaal, of course)
Jaheira – Will respect her abilities, but finds her a bit stiff. Conversations about being a fellow half-elf? Will treat him in manner similar to PC: overbearing but with best interests at heart?
Jan – Cailean is amused by his schemes and curious against his better judgment. Jan will try to lure Cailean into some sort of practical joke against another NPC or PC?
Keldorn -- Keldorn is stuffy but not as much as Anomen...he might give him some fatherly advice about women. Especially seeing that Cailean's whole mess is a result of being manipulated by a woman. Of course, given Keldorn's recent history Cailean would probably be a bit leery of taking it.
Korgan – Cailean would not like his evil, bloodthirsty ways. Korgan might be impressed by his reputation until he learns otherwise or perhaps he’s skeptical of everything that’s been said about Cailean.
Mazzy – Cailean would respect her. He'd give her credit for trying to be halfling paladin sort when everyone says she's not up to it. Mazzy could interject into the Cailean/HD bard banter about being restricted from what you really want to do. Mazzy would think him a little foolish for the trouble he was in and gently lecture him.
Minsc -- Minsc likes to right injustice. Cailean needs to right one. Good hearted, certainly...amusing, maybe...a big lug?
Nalia – Cailean could needle her good-naturedly and she could give it right back. Nalia could also maybe be a little fascinated (the adventure part) yet appalled (all those poor people's lives ruined by drugs) by his story.
Valygar – Will be sympathetic to Cailean and vice versa. He knows what it's like to be hunted and have to hide, not to mention to be accused unfairly.
Viconia – Cailean will not get along with her at all. Although he does not worship Selûne himself, his mother is a priestess of Selûne and has been brought up to revere her. Viconia, besides being at religious odds, will find the fact that Cailean was duped by a woman entertaining. Maybe she'd have a comment during the confrontation to Masalina? Cailean is also wary of her being drow, since his father's elven family has been warring with the drow for generations.
Yoshimo – Cailean is extremely uneasy about Yoshimo seeing as he has had bounty hunters on his trail for the last several months. Yoshimo could suggest to the PC that turning in Cailean could be profitable. We give the PC the option to actually turn Cailean in for the bounty (maybe 2-3k gold) if Yoshimo is in the party. The party is approached by a bounty hunter, that instead of attacking you, has a talk with Yoshimo ("come on, we'll split the profits"). He's already just mused about it you (or not; this could be the first mention of it) and asks you to consider it (again?). If you accept, this guy carts Cailean away after a struggle.