Rinko is the youngest member of the Yume Tsukai.

Unlike her sister, Rinko is more active and gung ho and is always ready for action. Smart, yet impatient, she is usually the first to act when it comes to seeking out the nightmares that pop up. She claims to be a Super Elementary School Student.

(Source: Wikipedia) 

When people dream, they express their utmost desires and emotions within the confines of their mind; but when their strong emotions cross the border into reality, the dream can turn into an uncontrollable nightmare. 
Touko and Rinko are sisters known as "yume tsukai" (dream users), and their job is to take care of these nightmares. Using toys as weapons, the girls must both destroy the nightmare and return the dream to its rightful owner before the nightmare does any sort of serious damage. 
Have no fear, Touko and Rinko are here! 
(Source: Anime-Planet)

Released her first single "Jump up!" on August 24, 2005.

Takaomi and Arika meet a young girl named Tsugumi Yoshimoto who has been asleep since an incident that occurred when she was a child. What happened in her past? And can she be cured? The couple try to solve this mystery with the help of the Mishima sisters -- Toko and Rinko, who are 'Yumetsukais' (dream messengers) with the uncanny ability to control dreams. What is the truth behind this mystery? 
This is a prelude to Riichi Ueshiba's next work, "Yumetsukai." The Mishima sisters make a comeback as main characters in "Yumetsukai." 
(from Kodan Club)

An ordinary, quiet town is shaken by a series of bizarre happenings brought about by the darkness in human hearts. There are no solutions in the real world: Only "Dream Messengers" can find the answers. Dream messengers connect the fabulous with the real and control the power of dreams. Their leader, Touko Mishima, works with 8 other messengers using the powers given by an old spell and a toy made by Touko's father.
 
Touko, her younger sister Rinko, Hajime Tachibana, who loves young girls, and the six other Dream Messengers are given the names of days or stars - Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Raff and Keito.

Rinko Mishima