Though I hate plot devices this is one of those cases that I think Kubo can get away with it and should be credited for.
Remember Zabimarou's character? A monkey with a snake tail with a "split personality disorder". The monkey was perfectly loyal, the snake would belittle and insult Renji all the time. I know it doesn't sound like much of an excuse for this powerup but I'm a sucker for these little subtleties and in the context of Bleach I think it woks for the best. Hope Kubo will keep this up and won't push Renji away from the spotlight yet again.