Ran Ryuuren, titled Lord of Time and Overlord of past, present, and future, serves as host for the Blue Immortal, one of Saiunkoku's Eight Colored Immortals. As youngest son of the powerful Ran Clan, he belongs to the direct lineage alongside his older brothers Ran Setsuna (a collective name for triplets) and Ran Shuuei, plus his half-sister Ran Jyuusan. The clan bestowed the name "Ryuuren" upon him at age four, signifying his status as a genius worthy of bearing the Ran Clan Seal and becoming the Blue Immortal's vessel. This name replaced his birth name, marking his destiny as the immortal's host.

Ryuuren often appears in flamboyant, unconventional clothing accessorized with feathers, vegetables like pears, cabbages, or radishes, or roadside finds, though he looks conventionally handsome in traditional attire. He wields a yokobue flute, playing it frequently and discordantly, causing distress to listeners and wildlife. Despite proficiency with other instruments, he deliberately chooses this disruptive style.

He exhibits eccentricity, impulsiveness, and intellectual brilliance, acting on whims such as erratic trading or pursuing ghost stories, and communicating cryptically. This behavior isolates him socially until meeting Kou Shuurei, who tolerates his idiosyncrasies. He designates her "Soul Friend #1" and To Eigetsu "Soul Friend #2," later assigning Eigetsu's alter-ego Yogetsu as "Soul Friend #2.5." His relationship with fourth brother Ran Shuuei is initially distant, referring to him as "foolish elder brother #4," but warms after Shuurei's influence.

The Blue Immortal manifests as a separate, dominant consciousness within Ryuuren, suppressing his eccentric persona. This entity possesses a calm, emotionless demeanor, inhuman knowledge, and a sense of humor. It controls time, enabling prophecy and awareness of events across Saiunkoku without physical presence, interacting freely with other immortals, teasing the volatile Purple Immortal and acknowledging the Red Immortal's past assistance. Ryuuren’s family, particularly the Ran triplets, likely knows of this dual existence. Clan protocol mandates obedience to Ryuuren over even the clan head when the immortal is active.

Academically, Ryuuren excels, achieving second place ("Bougen") in the Imperial Exams despite minimal study. His exam response rejected service to the Emperor—contrary to the prompt—suggesting he could have ranked first conventionally. He avoids official posts, honoring a family agreement to wander the country post-exams. During travels, he earns the moniker "Dragon Flute Gambling Master" through card games. Though disinterested in politics or leading the Ran Clan, clan elders consider him a marriage candidate for Shuurei.

He intervenes in critical situations despite his observer role as host, such as delivering the governor’s seal to Shuurei and Eigetsu in Sa Province, citing danger to his "Soul Friends." His martial arts skills, developed from admiration of nature’s power, and notable physical strength (e.g., throwing doors off hinges) aid these actions.

His prophecies, delivered as the Blue Immortal, shape events for other immortals, like the tale of the Rose Princess concerning the Red Immortal. The immortal’s knowledge burdens Ryuuren, requiring genius-level cognition to process past, present, and future across worlds. The Blue Immortal’s true name may be "Ran Ryuuren," inherited by each host.

Relationships define key aspects of his development. He deeply cares for half-sister Jyuusan, though she disparages him. Shuurei’s acceptance allows him to form genuine bonds; he appreciates her cooking, once proposing she become his personal chef. The Blue Immortal shows fondness for Uu Uu, comparing him to a doll.