Tenacity is not an established ability in the official Pokémon games, so I will describe an ability called Sturdy instead, which has a similar connotation.
Sturdy is a defensive ability in the Pokémon series that has a substantial impact on a Pokémon's survivability in battle. Initially, in generations prior to Generation V, Sturdy only protected a Pokémon from "one-hit KO" (OHKO) moves like Fissure and Horn Drill. However, from Generation V onwards, Sturdy received a significant upgrade.
Effects: 1. Survivability Enhancement: Starting in Generation V, Sturdy allows a Pokémon to endure any single attack that would otherwise knock it out when it is at full HP, leaving it with exactly 1 HP. This ensures that a Pokémon with this ability can never be taken out in one hit as long as it starts from full health. 2. Strategic Use in Battle: This effect makes Sturdy an excellent