A Kett Prefect is significantly taller that the Chosen and Ascendant. They are dressed in spiked armor with what appears to be flowing tarp behind them. A Prefect's voice is somewhat high-pitched, perhaps done to give them distinction over other Kett, most notably the Archon.
In battle, a Prefect would utilize a round, drone-like item to project impenetrable barrier around herself. The Prefect will then be impervious to any damage until the barrier is taken down, which creates a brief time window for Ryder and his/her companions to inflict damage until she restores the barrier. Being a high-ranking Kett, she is capable of indefinitely summoning lesser Kett to fight for her.
Mass Effect: Andromeda Edit
At least five Prefects are fought in the game's singleplayer campaign; The Cardinal in the Kett exaltation facility on Voeld, the Valiant and her Decimation squad on Ark Leusinia, the Invictor in Eos' Kett base, and two unspecified Prefects, one in the Kett mountain base on Voeld and one before the fight with the Archon on Meridian.
The three named Kett appears to show different responses when encountered by the Pathfinder's team. The Invictor behaves in domineering manner as Ryder challenges her, while the Valiant shows relentless behavior when pursuing the damaged Asari ark, and the Cardinal being somewhat cowardly in personality, to the point of pleading mercy to Ryder for the sake of her exaltation facility.