Parasect is an orange, insectoid Pokémon that has been completely overtaken by the parasitic mushroom on its back. It has a small head with pure white eyes and a segmented body that is mostly hidden by the mushroom. It has three pairs of legs, with the foremost pair forming large pincers. The fungus growing on its back has a large red cap with yellow spots throughout.
The insect has been drained of nutrients and is now under the control of the fully-grown tochukaso. Parasect can thrive in dank forests with a suitable amount of humidity for growing fungi. Swarms of this Pokémon have been known to infest trees. The swarm will drain the tree of nutrients until it dies, and will then move on to a new tree. Its former signature move is Spore.
Parasect first featured in The Problem with Paras under the ownership of Cassandra as a very weak Paras. Cassandra needed it to evolve into a Parasect so she can use the giant mushroom to aid in her medical research. After it battled with Ash's Pokémon (albeit, all of them except his disobedient Charmeleon not trying at all), and a battle with Team Rocket, Paras evolved.
In the Gold, Silver & Crystal chapter of Pokémon Adventures, Crystal uses a Parasect nicknamed Parasee which it first appeared in Slugging It Out With Slugma, extensively in her Pokémon capturing, using its Spore attack to put targets to sleep. Her Parasect is seen to be capable of increasing the radius of its Spore attack to at least 3 kilometers. In Lively Larvitar, the origin of Crystal's Parasect is revealed: it, along with Crystal's Natu, Hitmonchan, and Cubone, were wild Pokémon living in Mt. Mortar and had fought with a local Arcanine that had gone berserk from a severe eye injury. Parasect was able to create medicinal spores to help Arcanine heal and Crystal recover from her fractured arms. In Lugia and Ho-Oh on the Loose (Part 1), Crystal reveals yet another ability of Parasect: it is able to use the powder attacks it possesses to combine into a wider range of spores, including one for corroding metals.
Parasect can be seen as a counterpart to Shiinotic. Both share the Grass type and the same base stat totals, both can learn Spore, and both appeared as trial Pokémon of the Lush Jungle in Pokémon Sun and Moon depending on the game version, with Parasect being found in Pokémon Sun and Shiinotic in Pokémon Moon.
The fungus on Parasect is identified as tochukaso, an endoparasitoid that replaces the host tissue and can affect the behavior of its insect host. The base insect is a deformed version of what is probably a cicada nymph, the parasitic fungi having caused a form of neoteny.