Decided to read one of the earlier Bleach volumes and I found myself completely thrown off by Renji’s first appearance.
Like, the first time he appears in the series he claims to have been adopted into the Kuchiki Household, he strangles Rukia when trying to capture her, he plays around with the…
Renji didn’t say that he had been adopted by the Kuchiki family, that is a translation mistake. He said that Rukia was from the Rukongai, the same as him, and that she had been adopted by the Kuchiki family.
You can check in the first page in which Renji appeared in the manga: http://www.mangapanda.com/94-505-1/bleach/chapter-51.html , that Renji is worried about their mission of capturing Rukia. He comments that that is not the job of a shinigami. He looks worried, not agressive. So, no, Kubo didn’t change Renji’s personality later, because Renji always was like that.
What we saw after this was Renji trying to intimidate Rukia in order to make her go back to SS as soon as possible, because he didn’t want her to make the situation worse for herself. And this is confirmed by Renji himself 4 chapters later: http://www.mangapanda.com/94-510-9/bleach/chapter-56.html . He was very worried about Rukia’s sentence. Simply reading this people should understand what Renji was really doing and why he was acting like that. Kubo likes to create false impressions, but it’s not so hard to understand what really was happening.
Renji shows with this that he knew Rukia very well, knowing how she would act in that situation, trying to protect the person whom she had given her powers. So by this we can deduce that Renji and Rukia had a close relationship.
Rukia and Renji hadn’t talked to each other during more than 40 years, so it’s normal that Rukia would be very surprised when she saw him, so she even said his surname to confirm that it was really him who was talking to her. If anything, this shows that Kubo already had the matter of Renji and Rukia’s separation thought.
I was thinking about something…In the recent chapters, Renji and Rukia were reluctant to raise their swords to each other because they are ‘friends,’ but yet Renji didn’t have a problem sparring with Chad at Urahara’s training ground, and Rukia didn’t have a problem sparring with Orihime in Soul Society. Still, Renji and Rukia had this nervous tension while they were facing each other, reluctant to raise their swords, as if they were afraid of hurting each other.