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.
Effect Characters
These characters give effects when interacted with
Frog
A frog which gives the effect of the same name when interacted with. Two can be found in the game.
Non-Effect Characters
These characters do not give effects when interacted with.
Poniko/Uboa
See: Poniko
A girl that changes dramatically when a light switch is flicked
Monoko
See: Monoko
A monochrome girl with pigtails who changes when stopped with the stoplight effect
Monoe
See: Monoe
A monochrome girl whose face becomes a full-screen event.
KyuuKyuu-Kun
See: KyuuKyuu-Kun
A strange creature rubbing a pole
Seccom Masada
See: Seccom Masada-sensei
A cross-eyed man thought to be Madotsuki's piano teacher