Club: Toosaka Rin Fanclub

Pseudo-Servant. Basically, when a Heroic/Divine Spirit uses a human body as a host instead of an artificial vessel. It is actually preferable in some cases if the human host possesses greater similarity to the spirit than the Servant vessel of any one class. For Divine Spirits, it's always the preferred option - a Divine Spirit summoned outside their sphere of influence is nerfed to hell and back unless they abuse some loophole or other*. Using a compatible vessel that already has a basis for existence in the time and place of their summoning is one option. (Apparently, the body doesn't even need to be from that time period, just human enough to count - though not a Divine Spirit, Zhuge Liang is summoned using Waver Velvet as his vessel... in Ancient Rome.) It should be noted, though, that there's not a lot of information about Pseudo-Servants and about summoning Divine Spirits available, and what is there is phrased vaguely in typical Nasu fashion, so things mentioned here may end up inaccurate when the Babylon chapter of FGO hits us next month.

* A few loophole examples: Amaterasu had grown so close to humanity via her offspring (that is, every Japanese ever) that she incarnated and lived a mortal life; as a result, Tamamo-no-Mae, an aspect of Amaterasu and not at all a human soul, can be summoned as a Servant under certain conditions. Stheno and Euryale embody concepts that humans can envision, identify, and comprehend more easily than more abstract things such as natural phenomena (that is, "a desirable woman" compared to "personification of storms" or "ruler of the afterlife", things that either are difficult to understand or have never been encountered and so remain a complete mystery to living humans). Because of that, an artificial vessel can provide sufficient compatibility and they are far less nerfed (ironically, this means little as their combat ability is non-existent and they end up more of a threat when summoned as Servants than they would be as their full power divine selves). Finally, Artemis can summon herself alongside Orion and retain quite a bit of her strength, by the power of love. Yes, really; she wants to protect her hubby so much that she will bend the laws of the Universe to do it.

j/k