When running Baldur's Gate Expanded Edition to play the original Baldur's Gate 1 campaign (as opposed to the original edition of Baldur's Gate engine from the 1990s) you'll have access to the Find Familiar spell from character generation. Anyone playing any mage character other than a Diviner (who can't use conjuration/summoning spells) can consequently get a magical familiar companion from the very start of the game. (Hopefully this is still the case with the even newer re-released editions of Baldur's Gate Enhanced Edition that just came out recently as well, but we're not sure.)
Like the faithful magical animal companions in dozens of "casual" adventure or hidden object games, these faithful companions will stay with you and help with various tasks, and each has its own set of unique abilities.
Many Baldur's Gate RPG'ers overlook familiars because they are not fighting machines (though the Monty Python sorcerer "Tim's" line "this is no ordinary rabbit" still applies even to the fluffiest of them).
However, familiars are more powerful for a pacifist run than they would be on a combat-heavy run, and can easily constitute a "secret weapon", both because of their special abilities, and because they add another party member who can help you with a lot of miscellaneous tasks without causing any xp rewards from nonviolent quests to be reduced. This is because familiars don't level up on their own, so don't divert any share of xp divided away from your other character(s).
The main caveat is you have to be careful to protect the familiars: They aren't built like tanks, so they have to be kept out of danger for the most part. Fortunately, they can be protected by talking to them and asking you to let them pick them up so you can put them inside your pack, where they will be safe until you bring them back out again...
There is one unique familiar for every individual alignment in the game (e.g. lawful good, chaotic good, true neutral, chaotic neutral, etc.)
The familiars we experimented with in pacifist runs were:
THE FAIRY DRAGON
If you play a Chaotic Good alignment mage, casting the "Find Familiar" spell will present you with a Fairy Dragon, a friendly flying creature about the size of a cat.
In our version of BGEE (Baldur's Gate Expanded Edition) this was the most overpowered familiar, since it can cast the powerful 10' invisibility spell. This spell (normally a tier 3 mage spell that we never found available in any shops in BG1) turns you, the dragon, and any other allies within 10 feet invisible for up to 24 game hours. As with all invisibility enchantments, you can't grab any treasures or items, talk to anyone, or open locks without breaking the spell— but it's still extremely useful for nonviolent players, since it won't ever fail like stealth skills can, and can be used on an entire group instead of just singly by a thief.
Caveat: Exactly what abilities your familiar has might vary depending on which edition of BGEE you have. We saw some accounts indicating that the Fairy Dragon's invisibility spell got taken away in some versions of Baldur's Gate Expanded Edition(!). If so, the fairy dragon would remain a charming creature, but not the most powerful of the bunch. Also, while this spell is extremely useful, a pacifist run through BGEE doesn't critically rely on it, since any normal mage can get the level 2 invisibility spell after a few level-ups.
THE FERRET, RABBIT, AND CAT:
The Ferret accompanies a "Lawful Neutral" alignment mages who cast the Find Familiar spell. The Ferret is the best pickpocket, with 75% skill in this ability, as well as 40% in stealth, and 20% in detecting traps. It also has at least some skill level in opening locks, though the in-game description doesn't say exactly how much. Like the other familiars with thieving skills, the ferret can only pick the locks— it can't actually open the lids of chests and take items. However, it can pickpocket items, and then be talked to afterward, to ask it to hand over the items. If you wanted to go solo, then playing a solo Enchanter with a Ferret familiar to pick pockets for you might be one of the best choices, since you could cast invisibility on yourself, cast charm on foes you want to steal an item from, and have the Ferret pickpocket the charmed foe without having to split the XP equally with a thief. (Note: The reason this can work well is that a Charmed foe won't become hostile after a pickpocketing attempt fails, so you can repeatedly use the pickpocketing ability on the same Charmed target over and over again until it succeeds— unlike with a non-Charmed NPC, who would become impossible to pickpocket the moment they turned hostile to you after the first failed attempt.)
Perhaps the cutest of the familiars, but one with rather modest skills, the rabbit accompanies "Neutral" alignment mages: The Rabbit's description doesn't mention pickpocketing or lockpicking skills, but it still has them to some degree. Its relative strengths are its 50% detect traps ability, and it also has modest stealth abilities, but they're not powerful enough to justify the risk of putting it in dangerous situations where the stealth might fail. (However, when it comes to being white and fluffy and hopping around through flowery meadows, it excels.)
The cat accompanies a "Chaotic Neutral" alignment mage. The cat has at least some ability to pick locks and pick pockets, but its main strength is its stealth skills: It has 99% skill in stealth. However, like most cats, it isn't in any particular hurry to obey its caretaker, and seems to have the slowest movement speed of any of the familiars. (Even if you're patient, the major problem with this is that it will put the cat in greater danger when its stealth skill fails, since you can't equip familiars with special equipment like the Boots of Speed.)
Also: 99% stealth skill is very good, but not quite as powerful as it sounds, since in Baldur's Gate (and similar games based on AD&D rules), "modifiers" are applied to the continuously re-checked stealth rolls so that stealth still actually fails intermittently even when the skill score is well over 100%.