Reuniclus is a rounded cell-like Pokémon. Unlike its pre-evolutions, it now has visible limbs and ears, as well as a fully-formed body mass. Its light green body mass is surrounded by a translucent, green gelatinous membrane. It has a large round ovular head with a crease running down the center to the tip of its mouth. Its eyes are ovular and black, and its mouth is triangular-shaped and red with a visible pink tongue. Its internal body mass is relatively small with short stubby paws and feet. Despite this, it has four round particles of matter floating next to its paws. On the outside, these particles form long arms and hands with three finger-like digits on each. The gel-like membrane forms a rounded tail-like appendage at the base of Reuniclus's body mass.
It was officially revealed during the August issue of CoroCoro along with other six Pokémon on July 11, 2010.
Reuniclus, along with its pre-evolutions, are the only Pokémon that can learn Thunder but not Thunderbolt.
Nucleus (the center of the cell), can be spelled from Reuniclus' name by getting the letters N, U, C, L, E, U and S.
Like its pre-evolutions, Reuniclus is based on a cell, or single celled organism. The objects inside its membrane-covered body resemble organelles. Reuniclus also bears a strong resemblance to a homunculus, due to its larger ears, mouth, and hands. It also resembles an amoeba.
Reuniclus' name may be derived from "reunion" since it is a combination of Solosis and Duosion, and "nucleus", as of the whole Solosis family being based on mitosis and cells.
Its name could be a combination of 乱 Mamoru disorder, and ホムンクルス homonkurusu "homunculus", seeing as how it's a small humanoid figure in jelly.