how does rachel's character come in all of this?

Great Synopis! I am glad that Carter avenged both his brother and his niece.

please tell more. you made me totally understand a good portion of the movie...

I beg to differ, and I am basing this correction on the film and it's dialog.

Doreen's father Richie worked with Johnny who worked with Cyrus. Cyrus made porn with Johnny. Doreen and Johnny always seemed cool, he was nice to her and stuff. Then Johnny made an offer to Doreen to go to a party with this other girl that Richie knew named Geraldine. Doreen was excited, since she's young and influencable she looks with awe at Johnny (a barman, and she a young clubgoer?) and feels special to be invited. Johnny gets Doreen drunk on purpose and supposedly mixes some kind of drug in her drink (see visible State of beyond drunkenness while entering car, and Dialogue between her and Jack Carter later in the movie along the lines of 'I don't remember much' 'I couldn't move much' which emphasizes the use of some kind of date-rape drug)
Apparently this isn't the first time Johnny proceeds like this to have a porn video with Geraldine and some rather unsuspecting girl.
Geraldine keeps going with it because she needs money, and Johnny continues to have sex with her regardless of how much she asks to leave and to stop, which means he raped her (see dialogue 'she trusted you, and you raped her' between Jack and Johnny just before Johnny dies)
The entire thing was filmed and transfered to a disc. One thing let to another and Richie got his hands on the disc from Geraldine who felt bad and thought Richie should know what Johnny is like AND what happened to his daughter by people he trusted enough to have her hang around them.
Richie sees the video and sees what happens to his daughter.
Eventually Johnny finds out that Richie has the disc BECAUSE Ritchie tells them he'll go to the police with it so he goes to Cyrus to tell him to go hunt down Richie with the help of a few tough guys, supposedly as it contains audiovisual evidence of a rape having taken place with him as the rapist.
Richie is then tortured because he refuses to tell them were the disc is. They are all afraid Richie might go to the police so they kill him and set it up to make it look like a DUI car accident. This is why in a later scene in the movie that those same tough guys sneak into Gloria's (Richie's wife/Doreen's mother), they want the disc, they make it look like a burglary, but do not find it as it is hidden somewhere only somebody close to Ritchie (e.g. Jack Carter) would find it.

I personally thought it plain and simple to understand Doreen has been raped.
Shes awfully closed up to people that know her, even her mother complains to Jack in a conversation that 'she's gone and I don't even know where she went'.
Plus she's beautiful, young, and trusts the wrong people. Just the kind of prey Johnny and Cyrus need for their movies.