The Assistant is a Basic Card, but it explicitly counts as a Creature Die at the same time. The Portal card belongs to the Basic Cards as well, but its faces are akin to those of spells: there are no faces that show creatures, rather it has one face that provides Quiddity and the portal immediate effect on the other five faces similar to e.g. the Growth spell cards.
When the Strong Devotee of the Holy Query with burst scores, it permits you to capture one Spell Die that costs up to four Quiddity. Discriminating Incantation allows you to capture a Spell Die from the Wilds for free when you cast it. Does the Portal count as a Spell Die in this vein?