In the Shin Megami Tensei franchise, Tetrakarn creates a barrier on all allies that lasts for a single turn, which protects them from physical attacks, subsequently reflects them back to the attacker, and costs the attacker all of their Press Turn icons in games that use the Press Turn system. Generally, the user's related attack and defense stats are calculated in the process, and buffs, such as Tarukaja and Charge, are also put into consideration. In terms of enemy AI behavior, they are generally unaware of the shields cast onto the player party members when they attack.
In Shin Megami Tensei V, Tetrakarn protects one ally, and specifically grants a repelling resistance to Physical skills, not broadly Strength-based: the shield is based on the element, not the stat used in the attack. The shield overrides the user's stronger affinities, such as Drain. Additionally, the shield's reflection ignores the attacker's resistances and any shields and pierces through. Tetrakarn cannot protect against Piercing attacks. Tetrakarn cannot stack with other shields, such as Makarakarn and Elec Block, and will override them if used. Attackers cannot miss or land Critical hits on either the shielded targets, nor the attacker it's repelled towards. Repelled attacks cannot miss and can never land Critical hits. Attacks with secondary effects, such as debuffs or ailments, are sent back at the attacker as well. Draining attacks, such as Eat Whole, are converted into regular damage-dealing attacks. Multi-hit skills directed towards the target with the shield will always hit once, skipping any other attacks that were directed towards them.
In the Persona series, Tetrakarn erects a barrier on 1 ally, which protects them from 1 physical attack, and subsequently reflects it back to the attacker. (This applies only to Persona 4 and 5 -- in Persona 3, Tetrakarn and Makarakarn are applied to all allies.)