There are many different characters in Yume Nikki, some more reactive than others. However, as opposed to patterns in the background, characters will either move, react to Madotsuki or do both. Note: None of the characters, excluding Madotsuki, have been given official names. All dream characters were named by the fan community.
Effect Characters
These characters give effects when interacted with.
Shitai-san (Corpse)
A dead man on the street. Interacting with him gives the Stoplight effect, an obvious reference to traffic accident deaths.
Medamaude
A hand with an eye on its palm in the Eyeball World. Interacting with it gives the Medamaude effect.
Frog
A frog which gives the effect of the same name when interacted with. Two can be found in the game.
Ghost
Small one-eyed ghosts who give ghost themed effects. The cyclops ghost in Dark World has no effect, and instead follows Madotsuki. The ghost without a face in the Sewers gives the Nopperabou effect, and the ghost with the pursed lips in the Ghost World gives the Triangle Kerchief effect.
Mini-Lamp
A small street lamp with feet where a pole would be. Found in the Lamp World. Interacting with it gives the Lamp effect.
Mouth Creature
Usually appearing where there is a small blood puddle on the floor, these characters are simply a large round creature with a large mouth and hair. Interacting with one gives you its respective hairstyle. Specifically the effects that can be obtained from these are Long Hair, Poop Hair, and Blond Hair.
Non-Effect Characters
These characters do not give effects when interacted with.
Poniko/Uboa
See: Poniko
A girl who might change dramatically when you turn off the light switch in her house.
Mafurako
See: Mafurako
A girl with hat and scarf.
Monoko
See: Monoko
A monochrome girl with pigtails who changes when stopped with the Stoplight effect.
Monoe
See: Monoe
A monochrome girl whose smug face becomes a brief full-screen event.
Kamakurako
See: Kamakurako
A girl who sleeps within an igloo of the Snow World
KyuKyu-Kun
See: Kyuukyuu-kun
A large red smiling creature rubbing a stair rail in a hidden passage in Number World. 'Kyuu-kyuu' is a sound effect for 'rub-rub', an obvious reference to his rhythmic rubbing of the stair banister.
Seccom Masada
See: Seccom Masada-sensei
A googly-eyed humanoid alien in a colorless spaceship. He makes musical sounds when interacted with, and stands in front of a window at a control panel that looks like piano keys. Thought by some to be Madotsuki's piano teacher.
Martian
A one-eyed blue monster that can be found in the tiny hole on Mars using the Midget effect. It seems to be crying and talking to it makes a piano sound, stabbing it with the Knife effect will leave scars on his foot and change his tear color. He is also called by some fans Mars-san (Mr. Mars). Some theory he is crying due to being the last of his kind and misses his family and connected to the creature living in the pond near the tree where you get the Witch effect, and that it is his youngling that was sent to Earth seen here. Mars-san resembles the lamp in Madotsuki's Room.