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Looking at the Skill's description, there's five more Slots available.
{Divine Spiritual Vessel Guardians: 5/10}
Guardian 1: [Player: Titan]
Guardian 2: [Player: Achlys]
Guardian 3: [NPC: Titania]
Guardian 4: [NPC: Nieve]
Guardian 5: [NPC: Acorn]
Guardian 6: [Not yet Chosen]
Guardian 7: [Not yet Chosen]
Guardian 8: [Not yet Chosen]
Guardian 9: [Not yet Chosen]
Guardian 10: [Not yet Chosen]
I don't really know if I can get more slots than that though, the Skill itself doesn't say anything. So it might as well imply I can only add five more Guardians.
And that's totally fair, seeing how much power they obtain from the Divine Spiritual Vessel Awakening and all, it makes some sense indeed!
