Assist Trophies are items that, once picked up, cause one of a number of characters to come help your fighter in battle.
Straight out of Animal Crossing: New Leaf, Isabelle will cheer on the fighter who summons her and provide him or her with various foods. These treats will heal any fighter who touches them, so watch out for enemies trying to snag your snacks!
Ghirahim appeared in The Legend of Zelda: Skyward Sword. He moves like an acrobat and will attack with both sword and knife. Attacks against him will not fell him, and he can even teleport back into action should he get launched off the stage.
This enemy hails from the Kirby series. After he first appears as a power orb, he then changes into wizard form and proceeds to turn the entire screen pitch black. The players, of course, get lost in the blackness – this includes the CPU characters, who will make mistakes just like human players.
Phosphora first appeared in Kid Icarus: Uprising. This cool beauty will soar through the sky at high speed and shoot lightning bolts. Attacking her will make her flinch, but you cannot defeat her.
Color TV-Game 15 was a home game console released in Japan in 1977. Of all the game-related elements in Super Smash Bros., this has the honour of being the oldest.
In Super Smash Bros., paddles on the left and right will bounce a "ball" back and forth across the stage. Any fighters who gets hit by either the ball or a paddle will take damage and get launched.
Midna first appeared in The Legend of Zelda: Twilight Princess. Her hair changes into a giant hand, then goes to grab any enemy fighters nearby and tosses them into the air. Like grabs by fighters, Midna's grab cannot be shielded against. Also, if there are no enemies close to her, she'll chase some down by teleporting into the action.
Ashley is from the WarioWare series. When she appears, she creates a dark zone around her, and any enemies caught in it will move slowly and continuously take damage. There will also be various... changes... that take place.
Finally, any food eaten inside the dark zone will damage the fighter instead of healing them.
Mother Brain from the Metroid series is an enormous Assist Trophy that fires vertically-tracking laser rays. Fighters caught in the rays will rack up numerous hits and damage, so it's better not to be in front of her. By the way, it's possible to defeat Mother Brain by attacking her.
An energy-based life-form that bears a striking resemblence to Samus. Dark Samus will fire high-speed and homing missiles at opponents and, if approached, will use strong attacks to launch enemies.
The good doctor from Brain Age likes to hang out off the stage and fling numbers at the fighters. When the numbers collide and equal 10, it will cause a giant explosion. You can change the direction of the number bubbles by attacking them.