My Theory....

So basically, my theory is that Manga boy was running an under age prostitution / escort ring (with Amber, Jenny and her friends) out of the night club. That is why Jenny lies to everyone about knowing Manga boy and hangs out in the bar. Might not be a constant arrangement, but the girls are providing some kind of service. As such, Amber likely had some conversations with many of her clients, talking about normal things. At least one of the conversations she likely discussed her bedroom and painting it. One of those people likely got arrested at some point and told the guy in prison what her bedroom looked like.

I'm thinking that someone from the club (customer wise) likely saw her walking home and decided to take advantage of the situation by abducting her. He later realizes that he can't let her go, so he kills her (by drowning her) thus all the symbolization of water in the series. We never find out who this person is because we've never been introduced to them.

The guy on the train that keeps looking at her. Likely recognized her from the bar (not necessarily a customer) and then couldn't believe how young she was when not done up in make-up etc.

This just seems like what was going on to me.

You Gotta Be in it to Win it!