Dear Sasha,
Kache Marpo, being the emanation of Hayagriva (which is the wrathful form of Avalokiteshvara) is an enlightened Dharma Protector. Due to his enlightened nature, his works and reach are limitless. He is not bound to only his own original function but can emanate to have various functions according with time and need of sentient beings.
Although very often Kache Marpo is classified as the entourage of Dorje Shugden, but I think it is more accurate to call him the minister of Dorje Shugden. For example, the Prime Minister have his entourage, and ministers. As for the case of Dorje Shugden and Kache Marpo, we can look at it as if in a movie, where we have the Best Actor and Best Supporting Actor. Skill wise, both are at par, but they play different roles. It is true for this case. On a more spiritual note, due to Dorje Shugden having stronger karmic affinity with us in this era, it would not be impossible that Kache Marpo take a supporting role to assist Dorje Shugden. After all, being enlightened they are free of ego, and it doesnt really matter who runs the show as long as sentient beings get the most benefit out of their works
As for Kache Marpo as a single protector, he is still practiced within the Nyingma tradition as Tsui Marpo, where he originally enthroned in Samye Monastery by Guru Rinpoche.
