Mascots:

Kirby- An adorable pink puffball from Dream Land who has the unique ability to copy the moves of his enemies. Has a series of games and appeared in all 3 Smash Bros. games as a playable fighter. His round body means he could star in many solid spinoffs, including Kirby's Pinball Land, Kirby Canvas Curse and Kirby's Tilt N Tumble.
Official site

Marill- Pokemon number 183 in the National Pokedex, it was known as Pikablu before its official name was revealed due to similarities to Pikachu. A Water-type which has oil at the tip of its tail to help it float. Evolves into Azumarill.
serebii.net Pokedex entry
Bulbapedia entry

Pikachu: One extremely popular electric rat and the main mascot of Pokemon. Its number is 25 in the National Dex. The pouches on its cheeks store electricity. Also available for play in all Smash Bros. Evolves into Raichu with a Thunderstone.
serebii.net Pokedex entry
Wikipedia article

Yoshi: A fun-loving dinosaur that lives on the aptly named Yoshi's Island. When he's not being Mario's beast of burden, he can swallow enemies and turn them into eggs for weapons. Comes in tons of colors.
Wikipedia article
Yoshi's Island DS official site

Stafy: A young prince living in the Tenkai(lit. SkySea) kingdom in the clouds. His adventures are varied in motive, but most involve defeating the main villain, Ogura. Joined by Stapy, his sister, in his 3rd and 4th games. Both have unique moves. Also appeared as an Assist Trophy (read: a useless one ) in Smash Bros. Brawl
Wikipedia article
Densetsu no Stafy 4 official site

Dewy: A tiny water drop summoned by the Tree of Rainbow Colors to stop the evil Don Hedron from polluting the land with dark water. Can change form based on the temperature.
Dewy's Adventure official site
Gamespot's review of the game (6.0)


Ice Climbers: In the blue parka is Popo, and Nana is in pink. Though more recognizable by their appearance in Smash Bros. Melee and Brawl, they starred in one game on the NES in 1985, Ice Climber.
Wikipedia article

Chocobo: A flightless bird similar to an ostrich that has been a staple of the Final Fantasy series, usually for transportation and light combat. Chocobo also has a series of spinoffs and sports a more cartoony look in them, like in Chocobo Racing(PlayStation) and Final Fantasy Fables: Chocobo Tales.
Official site(Japanese)

 

Demiru: The star of the little known Nintendo-published RPG Tomato Adventure which was never seen outside of Japan. Uses weapons called Gimmicks to attack
Wikipedia article
Tomato Adventure official site

Chao: Adorable creatures from the Sonic universe. You can raise them in Chao Gardens and teach them skills such as swimming, flying and running. They can evolve into Hero Chao or Dark Chao depending on who's been raising them.
Wikipedia article