Choices made by main player characters throughout the game can affect their alignment to the light or dark side of the Force.
Alignment has a hidden point value between 0 and 100, representing the extremes of dark side mastery and light side mastery respectively. Main player characters begin the game at 50, with dark side actions subtracting from that value and light side actions adding to it. All dark side and light side actions have a point value assigned to them.
When viewing character models on the character screen, current alignment is indicated on the meter to their left. When light side or dark side mastery is achieved, the mastery bonus is displayed in text above the character model. The background and their facial expressions and stances also indicate where they are on the spectrum of good and evil.
Main player character portraits also change as their alignment shifts towards the dark side of the Force: from top to bottom, appearance corresponds to alignment 41-100, 11-40 and 0-10.
Certain items cannot be equipped unless a character has progressed beyond a certain threshold to either side: items restricted to the dark side cannot be equipped unless a character's alignment is 0-40, while those restricted to the light side cannot be equipped unless it is 60-100.
Your alignment also affects the color of your Name Crystal.
Light side power costs increase the further to the dark side a character is (and vice versa).
Use of Force powers of opposing alignment does not affect a character's alignment.
Once your character has gained level 15 and become sufficiently aligned to the light side (75-100) or dark side (0-25) of the Force (signalled by a scene of Visas and her master), speaking to Kreia will open a dialog which allows you to choose to become one of three Prestige classes, analogous to each Jedi class, depending on your alignment:
|Jedi class||Jedi Prestige class||Sith Prestige class|
|Jedi Guardian||Jedi Weapon Master||Sith Marauder|
|Jedi Sentinel||Jedi Watchman||Sith Assassin|
|Jedi Consular||Jedi Master||Sith Lord|
However, your character's existing class does not restrict your choice. For example, a Jedi Consular can become a Jedi Weapon Master if light side, or a Sith Marauder if dark side.
|Jedi||Jedi Prestige||Sith Prestige||Light side||Dark side|
|Jedi Guardian||Jedi Weapon Master||Sith Marauder||Strength: +3||Damage: Slashing, 1-8|
|Jedi Sentinel||Jedi Watchman||Sith Assassin||Constitution: +3||Dexterity: +3|
|Jedi Consular||Jedi Master||Sith Lord||Wisdom: +3||+50 Force points|
Mastery bonuses aren't permanent; they're only granted when alignment is at either extreme of 0 (dark side) or 100 (light side). A dark side action results in a light side mastery bonus being lost (and vice versa), although it can be regained with appropriate action.
Although the text above your model on the left side of the Character screen only displays the mastery bonus for your current class, if you have both a Jedi class and a Prestige class you should actually be granted the mastery bonus for both. For example, a Jedi Guardian who becomes a Jedi Weapon Master is granted a total light side mastery bonus of Strength: +6, while one who becomes a Sith Marauder is granted a total dark side mastery bonus of Damage: Slashing, 2-16. However, a Jedi Consular who becomes a Sith Lord is only granted a dark side mastery bonus of +50 Force points, not +100.
Although this text also displays for Jedi members of your party if they achieve mastery, they aren't actually granted this bonus.
Your own alignment and actions can alter the alignment of some party members, particularly as you gain or lose influence with them. Like you, this can alter their appearance: