To Summon a person. . . a thing. . . is simply to separate it from its world.
To summon is Extraction
It is to separate a thing from its place.
A Severance. A Sunder.
To separate a person of their heart you simply open their heart to the void and. . . sclup!
Basically Summoner should be tied to the Void and whatever results from that such as the Drain motif.

They would have some good Control, unique Shield from Summoned Entities, good Damage, maybe a little Lifesteal. . . with poor Freedom. . . hence empty lol