Simisage is a simian Pokémon that is primarily green. It has a pointed, bush-like crest on its head similar to a pompadour. The leaves that grow on this crest are bitter. Its ears are broad with light green insides, and it has oval eyes and a black nose. Simisage has furry, white eyebrows and shoulder tufts; and a cream-colored face, chest, hands, and feet. Its hands have five fingers, and its feet have three toes. Its tail is long and tipped with leaves. The tail is lined with tiny, sharp barbs, allowing Simisage to use it as bludgeon weapons. Simisage are prone to violently lash out at people and Pokémon for no reason.
Simisage and its pre-evolution are the only non-starter Pokémon to have Overgrow as a legally obtainable ability, in this case, as a Hidden Ability.
Simisage appears to be based on a monkey and a tree; the fact that it is based on both may mean that it might be based on the legends of monkey trees or possibly a visual pun on leaf monkeys.
Its hairstyle may have been inspired by the bōsōzoku or yankee subculture. This pompadour-like hair, common in the 50s, contrasts with the hippy-like hairstyle of Simipour, common in the late 60s and the 70s.
It two contemporaries seem to be inspired by the three wise monkeys motif; Simisear has a connection to "hear no evil", and Simipour to "see no evil". However, there is no obvious connection between Simisage's appearance and the third, "speak no evil".