Important Note: In case it wasn't obvious, this article is about the Light Side version of the Sith Inquisitor, not the more commonly seen Dark Side version, who is Darth Nox.
Darth Imperius was the title given to the Light Side version of the Sith Inquisitor who operated during the Cold War between the Sith Empire and the Republic. Though the Sith Inquisitor is typically seen as a "Dark Side" character, Darth Imperius was consistently shown to be a compassionate and kind-hearted individual, much to the surprise of friend and foe alike.
Darth Imperius began as a slave who was sent to the Sith academy on Korriban after their Force-sensitivity was discovered. Placed under Lord Zash, Imperius harnessed their incredible potential, also being visited by the Force Ghost of their ancestor Aloysius Kallig, who would occasionally provide guidance to his descendant, acknowledging that his own failures had contributed heavily to the downfall of his lineage. Reclaiming their ancestor's mask and also taking on the title of Kallig, Imperius eventually claimed a loyal follower in Khem Val, who also saved him/her from Darth Zash's scheme to steal her apprentice's body for her own.
With Zash now indisposed, Kallig found themselves in a position to advance themselves in the Sith hierarchy. But it soon became clear that Kallig would still face obstacles, particularly from the arrogant Darth Thanaton, who despised Kallig and viewed him/her as an upstart. Discrediting Kallig and marking him/her for death, the Sith Inquisitor was forced to flee. It was around this time that Kallig also took on an apprentice, a former Jedi named Ashara Zavros whom they trained as a Dark Jedi.
Eventually, by bonding themselves to various Force Ghosts, Kallig became powerful enough to challenge Darth Thanaton and was able to best him, afterwards getting his seat on the Dark Council, along with the title Darth Imperius. From there Imperius planned to continue his/her attempts to reform the Sith Empire from within so that it better fit the atypical Sith's more benevolent vision. Darth Imperius also kept their promise to free the Sith ghosts that had bonded themselves to him/her, save for one who elected to stay with Imperius anyway due to not wanting to go back to his tomb on Hoth.
Personality and Traits
In contrast to the usual personalities of Sith, namely Darth Nox. Darth Imperius was a Sith with compassionate and heroic virtues, spite of working for the Sith.
Known for finding more humane approaches, from sparing enemies to working alongside Jedi, such acts would cause for many to distrust and be suspicious of the descendant of Kallig.
Avoiding Sith behaviour, Darth Imperius was a Sith in name and title, in actions, identical to Jedi and in a sense, even more, virtuous and heroic.
Though with these characteristics, Darth Imperius was still Sith but believed that the Sith and Empire represented something else, rather a belief of wanton cruelty and xenophobia, Darth Imperius realized what the Empire could truly be, instead of being limited by itself.
The idea to reform the Empire from the inside, shared with their Apprentice, Ashara was an often expressed idea.
Despite the benevolent nature of Darth Imperius, like all Sith, the majority of the descendant of Kallig's power was derived from the Dark Side.
Extremely talented through a myriad of force techniques(and many of them forbidden), showed immense prowess as both an assassin and sorcerer, capable of shielding the mind and using the most dangerous Sith magic. Techniques included, but not limited to, Force Lightning, Telekinesis, Force Walking, Force Storm, and many others. Darth Imperious was also an incredibly skilled duellist, capable of using an ordinary lightsaber to a dual-bladed lightsaber.
More than just a skilled duelist and user of the Force, Darth Imperious had a mind capable of resisting the mental powers of Force Ghosts, the Dread Masters, and even Valkorion - The Immortal Emperor himself. Whilst not the greatest feat, but rather among it, would be the act of allowing the Force Ghosts he had previously chained to himself, to become one with the Force, an ability rare even among the greatest of Jedi.
Though even this feat does not match the defeat of Valkorion as the greatest show and proof of the Dark Lord's power. It should also be noted that whilst Darth Imperius's abilities were virtually dark side orientated, unlike many other Sith before and after the Dark Lord, Imperius never became corrupted by the Dark Side, capable of even using light side techniques at the same time.