It evolves from Eevee starting at level 20 while the player has a Nuclear-type Pokémon in their party.
One of the only non-feral Nuclear Pokémon. It's theorized that its bond with its trainer when it was an Eevee allows it to stay in control.
Nucleon has a sleek gray body with four slender legs, each with a green three-toed paw. It has three light-blue eyes. Its muzzle and underbelly are white, and its tail and ears are slender green triangles detatched from its body. It has an ionising radiation symbol on its chest with three detatched green triangles, giving it the appearance of a bow-tie. Nucleon remembers the bond it had with its trainer and thus does not go feral upon evolution from an Eevee. The symbol on its chest can also fire out gamma rays which it defeats its opponents with. They are usually only found in urban areas under the care of trainers, though they can be occasionally seen roaming near irradiated landscape.
Nucleon does not share the same stat spread or base stat total with the other Eeveelutions. It has a base stat total of 530 (as opposed to 525), and its stat spread is 70/55/85/115/115/90 (as opposed to some combination of 60/65/65/95/110/130).
Nucleon has the highest base Special Defense of all naturally Nuclear-type Pokémon.
Nucleon is the first Nuclear Pokémon listed in the PokéDex, as Nuclear forms are not listed.
Ironically, it is impossible for Nucleon to be the first Nuclear Pokémon a player has interacted with besides trading - Theo only acquires one after Vinoville Town, and the player cannot have a Nucleon without first catching another Nuclear Pokémon themselves.
Nucleon is also the only Nuclear-type capable of having its Hidden Ability.
Evolving into Nucleon will override all of Eevee's other level up-based evolutions, meaning that the player must remove any Nuclear Pokémon from their team before Eevee can evolve into Espeon, Umbreon, Glaceon, Leafeon, or Sylveon. Alternatively, Eevee can be evolved into those Pokémon before level 20.
Nucleon shares characteristics with foxes (especially the fennec fox), cats, and dogs. On Nucleon's chest is the international warning symbol for ionizing radiation. Nucleon has a third eye on its forehead, a characteristic mutants are often depicted with in fiction.
Nucleon comes from nuclear, nucleus (the center of an atom), or nucleon (one of the particles found in the nucleus) as well as Eon (an immeasurably long period of time, presumably in reference to the concept of evolution). Eon is also a suffix shared by all Eeveelutions and was Eevee's prototype English name.