In Death Note, the killer notebooks are a supernatural gift -- or curse -- that originate from the shinigami realm, a shadowy place of decay and chains. Ryuk acted as an ambassador of the shinigami, or at least a representative, but there was another major shinigami who was nearly his total opposite: Rem.
On one hand, Ryuk is an impartial observer as Light uses the Death Note to fight crime and remake the world in his image. Meanwhile, Rem accompanies the second Kira, Misa Amane, acting as her friend and guardian. But Rem's affection for Misa heart was turned against her, spelling her doom in the end.
Rem was a by-the-book shinigami, well-versed in the Shinigami King's rules and those of the Death Note. She had an associate named Gelus, a hapless shinigami who had fallen for a certain human girl: Misa Amane. Rem knew when Misa's day of death had arrived, and she watched to see how Misa would perish. A stalker was about to stab Misa to death when Gelus, to Rem's surprise, used his notebook to kill the stalker and save Misa's life. In so doing, Gelus betrayed an unspoken rule of shinigami and crumbled away. Rem then claimed his notebook and decided to meet this human for herself.
Misa and Rem quickly became friends once Rem handed over Gelus' notebook. Rem took a liking to Misa herself, and unlike the impartial Ryuk, took it upon herself to protect Misa at any cost, even at the risk of her own life. Rem explained the notebook's rules and allowed Misa to make the eye trade with her, and then Rem learned that Misa was a huge Kira admirer. Kira had slaughtered the criminal who murdered Misa's parents, and Rem watched as Misa made every effort to meet and team up with Kira.
Eventually, Misa got her wish and became Light's lieutenant, but the protective Rem made it clear to him that Misa was under her protection. But before Rem could kill L on Misa and Light's behalf, Misa was captured, though she was willing to die in captivity to protect Light's secrets. Alarmed, Rem allowed Misa to lose her Death Note memories and powers, then confronted Light and demanded a solution. Light had one, but his plan would eventually lead to Rem's downfall. From this point on, Rem would be Light's unwitting pawn, manipulated by her single-minded devotion to Misa.
Light needed to convince L that he and Misa were innocent, and to do that, Light turned himself in and gave up his Death Note memories. At the same time, under Light's command, Rem took his notebook and found a new owner: the greedy Kyosuke Higuchi. Rem became Higuchi's official shinigami partner and stayed with him until Light killed him to regain his powers and memories. During the time when Light and Misa had no memories or notebooks, they helped L investigate Higuchi, and Rem even approached Misa to help her zero in on Higuchi as Kira. Then, once Higuchi was captured and killed, Rem became a free agent, totally visible by several members of the Kira investigation team, including L.
Rem was still highly protective of Misa, and knowing that, Light asked Misa to resume operations as Kira, even with L watching closely. Misa was in immediate and grave danger of being caught as a Kira suspect, forcing Rem to act. At the cost of her own life, Rem wrote down L's real name, L Lawliet, and also killed Watari, or Quillish Wammy. Both victims died, and by extending Misa's lifespan, Rem died as well.
As she crumbled to sand, Rem reflected on how Light had actually surpassed the shinigami, being able to kill not just humans, but shinigami like Rem, too. Light quickly swiped Rem's notebook and feigned ignorance of what had just happened, and with Rem out of the way, Light became the new L and began his reign atop his new world, with Misa by his side. Ryuk, ever the neutral observer, thought little of Rem's death and continued to watch Light's gruesome quest unfold.
