Duosion is a rounded cell-like Pokémon. Like its pre-evolution Solosis, its inner body is pale green in color with a darker green, jelly-like membrane surrounding it. Its outer membrane is thick and has two rounded bubble-like protrusions on either side of its bottom half. An unattached pale green segment floats above its head. It now has what appears to be a developing torso, and two stubby arms. It has a face that is identical to its pre-evolution, but its head now has a seam running through the middle. It apparently has two divided brains.
Duosion is similar to Magneton and Klinklang in that more than one of its pre-evolved form forms together to make its evolution.
Duosion is based on a cell that is undergoing fission; as a result, its brain split into two.
Duosion's name is a combination of "duo" and "fusion," mainly due to its differing change and genetic appearance from its pre-evolution, Solosis. Its title, the Mitosis Pokémon, is sourced from mitosis, which is the process in which two eukaryotic cells split and share genetic material.