Viconia

Viconia is a Neutral Evil Cleric who can be recruited in all Baldur's Gate games. She's a poor melee fighter, but she does have high magic resistance. Her low strength prevents her from wearing a lot of heavy armour and shields, though her high dexterity offsets this to an extent.

In all three games, except the Throne of Bhaal expansion for BG2, Viconia will leave the party if your reputation reaches 19 or 20, regardless of whether she is in a romance with your character. It can be difficult to keep reputation below this level if you're playing a good party, as a lot of quests increase your reputation. Some other companions will object to having Viconia in the party, and paladins will likely leave altogether.

As Viconia is a drow and this race is generally despised by 'surfacers', allowing her to join your party will result in your reputation falling by two points. However, if you later remove her from the party, you will regain the lost reputation automatically.

Baldur's Gate 1

Viconia can be found in Peldvale, where you will have the option to save her from a Flaming Fist officer. The only way to do this is to kill the officer, but in this case you will not lose any reputation for doing so. Her wisdom is very low for a cleric (15), so she doesn't get many bonus spells.

Baldur's Gate 2

Viconia can be found in the Government District, where once again you will have to save her - this time from an angry mob. Her wisdom has increased to 18, which means she gets several bonus spells.

Romance

If your character is male and half-elven, half-orc or human, he can enter into a romance with Viconia in BG2.

Without giving too much away, there are two options for successfully romancing Viconia: you can either act confident and aggressive (characteristics which the drow approve of) or be sympathetic to her plight (which will annoy her at first but eventually she will relent).