So in Each game there was Garrett's Evil counterpart or foil.
In Thief 1 we are introduced to Artemus:
Only keeper whose stealth rivals Garrett's, Artemus is what Garrett could have been if he stayed with Keepers.
In Thief 2 we are introduced to Karras:
Genius who rebeled against his order, sadly there is no more I can tell about him
In Thief 3 we are introduced to Gamall, Garrett's Evil Counterpart
Gamall was Keeper Elder and Garrett was Initiate, both of them are former Keepers (and they are only two, Keepers assassinate rogue agents) and are only two beings outside Keeper Order that know about Keeper's involvement in events.
Garrett is professional who is unwilling to kill on a job, Gammal is contrast, she is ruthless and willing to kill anyone if they stand in her way, indeed, Garrett is master thief who dont want trouble and tries to forget about Keepers, Gammal is serial killer that wants power, and infiltrates keepers.
Both Gamall and Garrett are very resourceful and talented individuals.
While Garretts perception and skills in art of stealth are very good (he is implied to be best sneak they ever had), his knowledge about Glyphs is very limited, Gamall is complete opposite of Garrett, she is not very good at sneaking (she was scribe,not field agent), her mastery of Glyph magic makes her very powerful and dangerous being ( there is no being with greater glyph mastery,she is even more dangerous then Trickster).
Both of them are near mythical beings within The City and within Keeper Order.
Garrett, 'The One Eyed Thief', is world's greatest master thief, despised by most, feared by all, everyone heard about his exploits. Gamall is know as 'The Hag', a mythical serial killer, tales about her are so terrible that few even dare to speak about them at night, even though even fewer believe she actually exists.
Within Keeper prophecies there are two beings of great importance to Keepers, vital to their survival, or destruction: The One True Keeper (Garrett) and The Brethren And Betrayer (Gamall).
Bottom line: Gamall is what Garrett could have became if he turned to murderer instead of thief, OR if he stayed with Keepers long enough to find about dark secrets of their Order, or Glyphs.
It is interesting what character will be in Thief 4, we had a: Master Keeper, Genius who rejects his own Order and changes them, Dark/Evil version of Garrett.
It will be interesting if there will be another master thief, who was not involved with Keepers, showing that Garrett could have became what he is, even without Keepers.