All significant character relationships, ties or influences on Sylar.
At first pushed off as acquaintances it turns out the two have a very large history together.
Apparently according to Sylar, the two weren't always close -- Until Sylar lost to Nex. He still does show a tendency to push Nex's buttons, but its far more tame compared to the past. He'll constantly test his strength against Nex.
Their ideals do clash however, Nex won't stand down from Sylar, and Sylar won't always listen to Nex. The two go after their own goals, but share a mutual desire to alter the world around them leading them to join forces. He deeply respects him as a comrade, the two won't get in the other's way as far as goals go, likely due to a respect of each other's power. Their relationship however, is constantly tested by antagonistic forces.
Sylar's connection to the pair in the stand-alone resistance is explored in the 2nd Verse of Control Sequence.
Sylar doesn't seem to get along with Reiga at all, it may just be due to the lack of respect he has for him or because he seems in his eyes annoying since Reiga constantly points out his criminal records and thinks of him as a Demon who lacks any compassion. Reiga tends to find Sylar's behavior unnecessarily crude, leading them to constantly get annoyed with each other. Mockingly, Sylar refers to him as the 'Savior Child', chastising his strength and his fight against him and Nex as a member of the NOS. He will often make a remark to how boring he is compared to Nex and Sylar has also expressed he'll crush Reiga if he steps in his way.
The only thing Sylar does seem to respect in him, is his rebellious spirit, but he believes he's too weak minded for any true defiance.
At some point during his time in Taoreta, Sylar would meet these two struggling to survive in the lands of the Seithr-infested region. It's further explained in the Third Verse of Control Sequence how his relation with the two went.
Sylar and Komyo seemed to have met in Kanayama, but Sylar is so oblivious that he doesn't even recall it or recongnize her. When he does, he seems annoyed that she still sees him and Nex as the same people they were and warns her not to expect anything from them in the way of the world or Nex.
Sylar knows of Abel through his history with the Liberation Sector, their relationship is further explored in the 5th Arc of Rebellion Sequence and his Origin Story.
Midnigha met Sylar in Kanayama, their relationship is further explored in his Origin story.
Sylar knows Cerea through Nex, their relationship is further explored in the Arcs of Rebellion Sequence.
Cal was Sylar's former close colleague and one of the leaders of his former criminal in the land of Taoreta. It isn't known what happened to him or how they ended up splitting apart. Their relationship is further explored in the 6th Arc of Rebellion Sequence and his Origin Story.
Emiah was one of Sylar's group members, their relationship is further explored in the 6th Arc of Rebellion Sequence and his origin story.
Cetan has connection to Sylar, both being a part of the Illegal Forces as a whole-- though not a lot is currently known. Their connection is more deeply explored in the 6th Arc of Rebellion Sequence.