An Aggressive Rat

Question
QUESTION: Hello,

I currently have 5 rats and I love them all so much. One day as I was walking through a store, I noticed there was a rat for sale, only one left in the cage, and I felt bad about seeing her there all alone and had to bring her home to show her love and to have companions. I never gotten a lone rat before, I always had at least two rats when I have adopted them. The new rat, named Ellie was about 4 months old.
I brought her home and let her have her own cage, housing, and the works. I slowly started to introduce her to my other females and things started out great! Until one of my females automatically jumped her and started trying to bite at her face. I separated them and let them cool down and tried again to see if it was going to escalate and it did.
It has now been a few weeks and NOTHING has changed. The older rat, Sophie, has always been aggressive when she is near. Even when I let one out and the other is in another cage, Sophie will try to attack Ellie through the cage.
I'm not sure what to do to try and get this aggression out of Sophie. Ellie doesn't seem to show any aggression, but its like she knows Sophie wants to get to her and they antagonize each other.
So what I am asking is, how can I try to introduce Ellie and Sophie without there being any aggression?

ANSWER: Hi Alexis,
Tell me a little bit more about what Sophie does when she attacks or tries to attack Ellie. Does she puff her fur up?
Deb

---------- FOLLOW-UP ----------

QUESTION: From what I have seen, she does not puff her fur up. She just runs right up to her, not bothering to sniff her or anything, and attacks, without hesitation. It's like its become some kind of obsession. I know it sounds weird, but when Sophie is out of the cage and Ellie is in her cage, Sophie will try to chew on the bars to get to her and will constantly circle the cage. It's a little unsettling
But it is also strange to me too, because when I first got Sophie and a couple of her sisters, another one of my rats was quite rough with her, biting at her bum and trying to pull her around and now things are fine. It never got to a point to be too worried when I first got Sophie. Now it almost seems like revenge, like its her turn to go after the new kid at school.

Answer
I don't think Sophie is seriously trying to injure Ellie. Rats almost always puff up their fur when they are aggressive and meaning to hurt another rat. I think Sophie is doing this to show Ellie that she is boss. A rat can grab another rat's face to pull them around and make them do what they want. What does Ellie do when Sophie tries to grab her face?