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Sentinels are the special guards of Sakura Labyrinth. They are beings that were remodeled by BB.
They are the main core of each level of the Labyrinth. They are part of the Labyrinth itself.
Only girls who have good quality magic circuits and a “heart” can become a Sentinel, so it’s not possible to mass produce them like Attack Programs.
From among the Masters, Rin or Rani, and Jinako become sacrifices and are transformed into Sentinels.
When creating a Sentinel BB acts as an external terminal, and inserts her consciousness into the victim. The process ends up imprinting some of BB’s personality on the victim. Surely. Probably. For the sake of Rin’s and Rani’s honor, let’s assume that’s what happens.
Any girl who has been made into a Sentinel has had her spiritron core (their soul) sealed away by making it into part of the core of the Labyrinth, and furthermore all her five senses are used as building materials to expand the Labyrinth.
This means that changing the structure of the Labyrinth, or increasing the number of traps is a very painful procedure for the Sentinels.
Also, while the essence of the ego (the atman) is sealed away as part of the Labyrinth core, the more personal bits of the ego, such as the individual interests/preferences part of the personality, become avatar (bunshin off-shoots) which are called “Ego” (desire bodies). These “Ego” become managers of the various areas of the Labyrinth.
While this is digression, high level computational ability is necessary to change or reform the Labyrinth. Rin and Rani have enough ability to fulfill the basic requirements, but Jinako does not. This is probably the reason why her part of the Labyrinth is so plain and simple.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA