"Niji" is a 1995 single by the Japanese technopop band Denki Groove and is featured in the Eureka seven OST 2. Despite its only appearance being in the final episode, it is considered a popular song. Featured in the final episode of the anime series, Episode 50: Wish Upon a Star.
Renton Thurston is gearing up mentally and physically to rescue the girl he loves, Eureka, from becoming the next Control Cluster.
The song itself plays for 10 minutes and 55 seconds, so it is heard throughout the majority of the second half. The song "Seven Swell" in Eureka Seven AO was based off of "Niji".