Lieutenant Commander Ranul Keru was an unjoined Trill who served as a Starfleet officer in the late 24th century. He served as a stellar cartography officer on the USS Enterprise-E. He was the husband or life-mate of Sean Hawk until Hawk 's death on the Enterprise-E. Also, due to losing Sean, who himself was also a Starfleet officer, he decided not to accept the romantic advances of another officer aboard Titan: Ensign Kenneth Norellis.
He continued on the vessel for some time after Hawk's death, but by 2376 he took a leave of absence to return to Trill and serve as a guardian at the symbiont breeding pools at Mak'ala. Late in 2376, he aided Ezri Dax and Taulin Cyl in defending the pools from unjoined terrorists.
Sometime before 2379, he returned to the Enterprise, but transferred from stellar cartography to security. Later in that year, after joining Will Riker and Deanna Troi on their honeymoon, he joined the crew of the USS Titan as Chief of Security. After spending a portion of time in a coma, he handed over the duties of tactical officer aboard Titan to Commander Tuvok.
Keru's loss of former lover Hawk at the hands of the Borg made him initially abrasive towards cybernetically enhanced Choblik Torvig Bu-Kar-Nguv when the young man came aboard as a cadet. However, the two eventually found a working relationship which later blossomed into a friendship after the cadet graduated and was permanently assigned to the Titan. When the Borg launched their attacks in 2381, Keru sought out Torvig for his knowledge and familiarity of cybernetic based lifeforms, and called him a great problem-solver, a high compliment to a Choblik.
Torvig constructed several battle simulations boarding a Borg vessel; one such simulation ended with Keru being absorbed into the floor of the ship. Keru was also a member of the away team to board the shuttlecraft Mance and descend to the surface of New Erigol where they made contact with the Caeliar and missing Earth Starfleet Captain Erika Hernandez.
After being informed that the away team could not leave the surface of New Erigol, Keru and Torvig were assigned as one of the teams to scout the city of Axion. The two discovered an odd statue, and by striking the surfaces with hanging strikers Keru determined it to be a musical instrument of some kind. Although he quickly found appropriate ways to play it, he stopped himself, as he found he was losing himself in the performance. After the entire away team reassembled and shared their findings, Keru was one of the team members who agreed that Erika Hernandez should be considered compromised, and therefore could not be trusted. Keru was briefed on their escape attempt by Tuvok via a Vulcan mind-meld, and gave his tricorder to Torvig, who used it to beam Keru to the shuttlecraft Mance after Turvok initiated the diversionary tactic. He was successful in beaming to the Mance via its personal transporter, but, after restoring primary power, was stopped by the Caeliar Inyx. After having time to reflect on the mission, Keru remarked that they had now lost the element of surprise in any future escape attempts.
After Doctor Ree apparently attacked Deanna Troi, injecting a Pahkwa-thanh venom to stall her aborting fetus's growth, the other away team members confronted him, along with the Caeliar liaison Inyx; Keru was doubtful of his intentions originally. When Inyx informed them the Titan had escaped, and the others questioned it, Keru pointed out the Caeliar had not yet lied to them. Keru apologized to Dr. Ree, but shot him an anxious glance when Ree joked she was succulent. Some time later, Torvig led Keru through the catacombs of Axion, having detected a subtle shift in the city's quantum field, and located the Omega molecule generator powering the city-ship. When Troi returned to them, healed thanks to the Caeliar technology, Keru was the first to dismiss protocol and embrace the counselor.
Due to negotiation with Erika Hernandez and the Titan, the away team was eventually released from their capture. After returning to their ship, Torvig took some time to escape from his quarters and seek an isolated location in which to think; Keru found him in a remote forward compartment just above the main deflector dish, as his shared quarters with Ensign Worvan were unsuitable due to the number of questions he received from his roommate. Keru noticed that something was bothering Torvig, and he admitted an amount of anxiety due to the Borg, whom he had previously thought of as an abstract, instead of the reality now imposed upon him by the invasion forces. He found that he did not feel ready to face them in combat, and a greater fear of his own cybernetic components betraying him. Keru promised not to allow that to happen. However, the invasion was halted by the Caeliar's intervention, who dissolved the Borg Collective; the flood of emotions threatened to overwhelm telepaths, and Keru relayed orders to Dr. Ree to check on the Titan's various psi-sensitive personnel.