"The One Being Sought" (捜シ者) was Rei Ogami's older brother and a major antagonist in the series. He was the leader of a group of people with strong special powers known as the Re-Codes. He was killed by Ogami.
Appearance
When "The One Being Sought" was first introduced he was in his "Lost Form", he looked identical to Ogami in terms of both age, physical appearance, and manners to the point that Sakurakouji Sakura thought them to be twins. His coat and hair color, however, were bleach-white, which seperated him from Ogami in terms of appearance.
When he revealed his original form after obtaining the Pandora's Box, he was much taller, had longer hair, and carried a long scar over his right eye all the way down to his waist, showing just how much of an overall difference in age he has with Ogami.
Personality
Like Ogami, he is cold-hearted, ruthless and merciless. He also is shown to be always calm, rarely being intimated by his enemies. He is also shown smiling, but only when he has the upperhand in a situation.
When he returns to his original form, he is shown laughing and smiling much more due his confidence in his newly-regained power.
Background
"The One Being Sought" has no background on his childhood; it is known, however, that in his youth, he was under the guidance of Shibuya, from whom he learned his techniques. He was actually kind and very compassionate person, before turning to evil. It was also revealed that his name was not originally "The One Being Sought", but was dervied from his Code:Name when he was within Eden; Code:Seeker, otherwise known as "The Seeking One".
He was very respected within Eden, but they saw him as an upcoming threat to them, so Eden had him exiled with both mental and physical scars. He then vowed to become evil, since he no longer believed that there was any true justice to begin with.
Later on, "The One Being Sought" had traveled overseas to many foreign countries, building up his own group of power users; The Re-code. He had taken his younger brother Ogami with him as well becuase he had been transplanted with the Code:Emperor's left arm remains. He treated Ogami like a slave and continually abused and hurt him, but still kept him around becuase he had the Code:Emperor's power (which lead to conflicts with foreign powers becuase of the arm's value). He soon became acquantices with Rui Hachiouji, Kouji, and Yukihina later on and they helped in his quest for power.
Soon, "The One Being Sought" would return home with Ogami under their parents' care. It was unknown what happened, but an all-out war with the Code:Breakers and his group the Re-Code happened, and he killed his own mother in front of Ogami after she protected Sakura Sakurakouji because of her privileged status. It was known at the time that his true form had been sealed into Pandora's Box and he was forced into his Lost Form for quite some time afterwards. It was not shown, but he had taken Ogami with him after the events settled.
He soon started teaching Ogami how to live life (in a very strict and military-like manner). It was known that he had given Ogami his trademark black glove to contain the Code:Emperor's power and to allow him to be compassionate. Ogami would soon split ways with him after finding out the truth of what his intentions were. It is unknown if "The One Being Sought" actually tried to stop Ogami or not, but its possible he had let Ogami make his own choice by himself.
History
'He' personally asked Rui to look after Ogami, predicting that Eden will see Ogami as an 'unforeseen threat' as well, just as how Eden saw 'Him' to be. So he sent Rui to protect his younger brother. He also wants Ogami to find an 'everlasting truth' (probably about Eden), that even 'He' couldnt find, hinting that "The One Being Sought" still cared for Ogami.
'He' also sent Kouji and Yukihana to protect Ogami when Code:Emperor revived, knowing that Ogami needed as many allies as possible.
He hid a Re:Code with in Ogami's and Sakura's school to further look after them.
The seal he placed on Sakura was revealed to be for her protection in case she came close to dying. The seal was triggered in the hospital, after Sakura protected Ogami from Aoba, and he appeared to her and told her a bit about the December 32 incident. He revealed that he investigated it when he was a member of the Code:Names, and was ordered by Eden to be assassinated. He told her that the Four Founders of Eden held the truth to the incident.
Yukihina seeks to avenge "His" death by Eden's hands, and wants to be the one to kill Ogami.
Powers/Abilities
High Intellect: He has been shown to be a very cunning man, as he was able draw accurate conclusions from little to no information about a subject instantly. In fact, it was commented that he was thought to be able to read minds, however this is proven false as he was in just making conclusions based off of his observations and using his high IQ. He is also a skilled planner at thinking ahead as he was fully aware of Eden's influence and believed that Ogami would be endangered one day once his power became feared by Eden, so he had asked Rui; one of his most trusted allies, to protect him if the time ever arrived.
Lost Form
"His" Lost Form
Master Swordsmanship Specialist: As Shibuya's apprentice, he has been trained to be highly skilled in swordsmanship. He is so skilled that he is capable of being able to inflict severe wounds from a far distance without even drawing attention. He was able to defeat his younger brother Ogami who was the current Code:06 of Eden with little effort.
Enhanced Durability: He was able to take on Ogami's Azure Flames head on, while he was slashed by his sword instanteously and remained virtually unfazed by it despite the severe injury.
Enhanced Speed: He was capable of being able to avoid Heike's attack when he removed his limiter with no signs of being affected by the attack at all.
True Form
"His" True form
Immense Durability: Enhanced even further due to his power of Cell Regeneration, to the point where he is able to survive being burned by Ogami's Azure Flames continously without dying, an amazing feat considering how powerful the flames are and that they were fully awakened.
Immense Speed: Thanks to his power Absolute Space, he is able to avoid the most powerful attacks with little effort and teleport so fast that he can catch most, if not all, enemies off guard.
Immense Strength: Thanks to his power of Absolute Space, his strength is enhaced tremendously to the point that just throwing a few physical blows to someone of Shibuya's caliber was enough to leave Shibuya almost completely weakened. He was even able to create small craters with great ease and being to show just a glance at Ogami wounded him.
Special Powers
Techniques
Ryuuseisou (Frosty Iced Meteor): This technique is used exclusively by "The One Being Sought". He first has to unsheath his sword at extremely high speed, allowing the sword to be lifted, and it releases an attack in a spiral-like pattern that destroys all that surrounds himself. An example of the damage is that he was capable of doing was being able damage multiple targets, including two Code:Breakers, a Rare Kind, and two of his Re:Codes simultaneously before any of them could even react.
Sever: This technique is a swordless version of Ryuuseisou, cutting everything in a diagonal pattern. It can be used by "Him" multiple times.