Shell Psions are identifiable by their squid-like silver body and purple bumps. This is important for choosing which island to save as the psion will be present on all missions in said island, making the initial Vek preview vital.
As with most Psions except for the Psion Tyrant, the Shell Psion is encounterable from island 1. It grants all non-psion Vek the Hardened Carapace buff, reducing incoming weapon damage by 1 much like Abe Isamu's pilot ability.
The Shell Psion is the worst possible psion to encounter, as many mechs initially only do one point of damage. This becomes absolutely nightmarish on island 1 and can remain a problem on later islands with loads of alphas. If your squad cannot immediately kill them with one attack, spend your cores on movement and damage upgrades. Otherwise, spot which island has the Shell Psion and avoid it.
The only sitation where the Shell Psion is easy to deal with is with the Flame Behemoths. as the squad uses fire damage that bypasses Hardened Carapace. Push damage and spawn blocking also bypass the armor.