The tetra is an immobile field object intended to impede mobile progress. By defeating a tetra, the player gains a Tetra Seed card.
The tetra is very useful for defending in the early game, as it takes a long time for the enemies to defeat the tetras. Tetras have less defense growth than every melee unit except the Mantis, only moderate defense, has poor HP in the first place, and has an HP growth equal to the weakest gun units. This is, of course, combined with 1 speed, and 0 barrier. So any beam attack will yield full damage with one single point of total defense. Tetras also are on no side, so it is possible to trap a unit in their own tetras. and the tetra-placer can only place tetras adjacently and on the end of their turn. However, each adjacent square is protected by a tetra, so each attack from an enemy requires another tetra to be destroyed. To attack a unit from all four sides, the enemy needs to use at least 7 units to do it. Also, each Tri-Tetra creates three tetras, three different units. It is very possible to, instead of healing or using aggressive attack cards, just utilize tons of tetras. It is possible to use up to 48 tetras in one battle, which is especially useful in the Badlands for wasting each unit's cards. So it is extremely questionable whether tetra cards are absolutely useless or game-breakingly useful.