While it is not known to evolve into or from any other Pokémon, Wishiwashi can change forms with its SchoolingAbility if it has reached level 20. When Wishwashi enters a battle or ends a turn with its HP above 25%, it changes from Solo Form to School Form. School Form Wishiwashi will revert to Solo Form Wishiwashi if its HP is below 25%.
Wishiwashi is a fishlike Pokémon. It has two different forms depending on the situation.
In its Solo Form, Wishiwashi resembles a small fish. It has large, light blue eyes with irises that resemble puddles of water and dark blue eyelids. The head and tail are both white. There is a line of white teardrop-shapes on each side of its body with the teardrop nearest the head being slightly larger. The top half of its body is dark blue and has a short, rounded dorsal fin. The lower half of its body is gray with two pectoral fins. This tiny Pokémon is both the shortest and lightestWater-type.
In its School Form, Wishiwashi resembles a large fish roughly formed from various smaller fish with glowing eyes. Starting with its head, its body is divided into four bands that each becomes a slightly darker shade of blue. A ring of bright blue glowing spots lines the front edge of the two middle bands. The centers of its "eyes" are formed by a single white fish with glowing eyes for pupils. Surrounding the white fish is a dark blue recess, and at the rear edge of the "eye" is a raised white area for eyelids. Above each eye is a single, dark blue fish shape. Wishiwashi's mouth gapes wide open, apparently unable to close fully due to gaps on each side. Its lips in the front are lined with glowing spots, and many more spots dot the inside of its mouth.
On each side of the School Form's body, in the second band, there is a pair of dark blue fish-shaped recesses. Wishiwashi is able to fire fish-shaped torpedoes from this area. Its pectoral fins are the same color as its head and have three finger-like projections. There are glowing spots at the base of each "finger" and at the base of the fins themselves. On top of its head are two short projections shaped like fishtails, one behind the other. Additionally, Wishiwashi has a large dorsal fin that is darker around the base and has two glowing spots at the tip. The third section of its body has a ring of four spike-like fins around it. Three spikes encircle the tip of its tail, each with two glowing dots at the base. Dark fish shapes float freely a short distance from the end of each spike, all with two more dots on them. Finally, the tip of its tail is a rounded teardrop-shape with a single dot on each side.
An individual Wishiwashi is small, weak, and notably tasty. As a result Wishiwashi has numerous natural predators, including humans, Dewgong, Wailmer, Wailord, Wingull, and Pelipper. While Wishiwashi are known to move in schools, they're more often seen alone. Wishiwashi also gather when confronting opponents. It can call on its friends by shining its eyes seen as an SOS signal almost 25 miles (40 kilometers) away.
The people of Alola and Galar, however, still fear Wishiwashi and call it the "demon of the sea" due to its School Form. Even the fearsome Gyarados and Wailord are known to flee from it when it enters School Form. Wishiwashi assumes this form whenever a Wishiwashi is in danger or is bullied. Wishiwashi can concentrate their powers when assembled. The School Form's Water Gun is said to be more powerful than a Hydro Pump move. Should it get wounded in School Form, Wishiwashi will split into a regular school before fleeing.
Of all Pokémon with multiple forms, Wishiwashi's base stat total undergoes by far the most drastic change.
Solo Form Wishiwashi has the lowest base stat total of all Pokémon at only 175.
School Form Wishiwashi is the only Pokémon with a base stat total of 620.
Wishiwashi's Solo Form bears a resemblance to Pacific sardines. Its eyes' ability to light up may be derived from flashlight fish. Its bioluminescence, great abundance, and overall shape may also reference the lanternfish. Its School Form bears some resemblance to a submarine and the extinct Dunkleosteus.
Wishiwashi may be a combination of wishy-washy, describing the weak state of its Solo Form; wash, in reference to its Water typing; and 鰯 iwashi (Japanese for sardine).
Yowashi may be a combination of 弱し yowashi (weak, the classical Japanese form of 弱い yowai) or 弱々しい yowayowashii (frail) and 鰯 iwashi.