Essay
World Embryo is about lying and liars. Riku characterises the story as being a "gentle and bitter lie". Riku himself is a compulsive liar, who lies a lot of the time for various reasons.
Rena too is a liar. Her lie is that she's okay. She's not.
Rena and Youhei
When we first meet Rena, she's the :| to Youhei's :D. Where he is outgoing, genial and gregarious, Rena is stand-offish, blunt and coldly rude. The two of them have common ground in that they're both stubborn, determined and self-sacrificing, and presumably it is these traits that allowed them to build a strong partnership. And they are a good team and they trust each other implicitly. There's a scene in the second volume where Rena's been run through at the shoulder, thrown across a room, she can't see out of one of her eyes and she's holding onto a toddler while holding off someone who came into their headquarters and slaughtered other jinki users (and was the one who beat her up too). She's nearing her limit as to how long she can hold her transformation and yet she still believes utterly that Youhei will come any minute to help out. Because that's how they work.
Except that this time, Youhei doesn't come. Riku does, with Youhei's core (or weapon against the kanshu), and when Rena asks where Youhei is, Riku says that he's fine. He's not; Youhei's dead. The act of giving Riku his core began to turn Youhei into a monster, and Riku had to decapitate him so that the transformation wasn't completed. And Rena knows this and knows that she was lied to. Her initial reaction to seeing Youhei's body is the most natural emotion we've seen from Rena since the beginning of the series. She stares at the body, explains numbly that it should have been her that killed her partner and then she snaps. She all but throttles Riku screaming that it's his fault and she has to be physically pulled off Riku. And okay, this sounds pretty silly because Rena is a tiny Japanese girl, but we learn in a later chapter that Rena's strong enough to break Riku's neck without thinking about it, so her throttling him in a fit of furious misery is a pretty big deal!
To make matters worse, Rena actually was crushing on Youhei. He never knew and I'm not certain that Rena did either, but to the reader, it was clear that she did love him and that it's pretty likely that her feelings weren't requited.
All in all, Youhei was an Important Person to Rena, and his death utterly devastated her. It'd be entirely reasonable for her to mourn his death. The thing is, Rena ... really kind of doesn't. The next time we see her, she's coldly angry with Riku again, telling him that he has to come with her because it's what he owes Youhei. Considering that Riku is curled up on the platform of a Japanese train station, sobbing hysterically for his part in what he did, that's harsh. But it's also the first really big clue that Rena's cold, stand-offish surface persona is just that; a facade to give the impression that she's coping with things and that she's utterly untroubled by what happened.
But this isn't the only thing that Rena hides from others. The big thing she hides has to do with her past.
Rena's Emo Past
Throughout the series there is talk about "lost rebounds", or people who have traumatic amnesia as a result of a kanshu killing someone they're close to. The closer the lost rebound is to the dead person, the worse the amnesia. We see one case where a girl's boyfriend is killed and she practically goes catatonic. Eventually the person recovers in time, but they tend not to recover their memories ever.
Rena is a lost rebound and so we don't ... know that much about her past. We know that when she was very small, she travelled with a man who she calls "sensei" (but I'm told there's some connotation about her choice of words. Sadly I don't read this crazy moonspeak so I'll have to wait until someone translates it already) and that she has a great deal of respect for him. Someone very close to her died as a result of a kanshu attack, wiping her memories utterly clean, and two years ago she turned up at FLAG with a jinki and was assigned to Youhei's team. I think it's fairly reasonable to assume that it was Rena's mentor that died, and I assume that Rena's core came from him.
She has dreamed about her past once -- while drunk, no less -- and what's interesting is that while she dreamed of herself when she was about five or six, she talked about the source of infection. (This is important because the jinki users are looking for the source of infection to stop the progress of the kanshu virus) She asks Riku what she spoke about while she was asleep and Riku ... lied and said that she said she drank too much, to which Rena spazzed out like a winner.
Anyway, what's important about all this is that Rena has never told anyone that she's a lost rebound. The subject comes up when Youhei asks her if she's known anyone who's died from a kanshu attack, and Rena lies. She says that she doesn't know about any. This is a pretty clear lie because Rena knows she doesn't have any memories and later identifies herself to Riku as a lost rebound. This isn't a once-off event either -- no-one except Riku knows that she's a lost rebound and she's worked with these people for two years!
The circumstances behind the revelation to Riku were ... not so great. Basically in the latest scanlated chapter, Riku is caught up in some kanshu attack that exposes his memories to the kanshu ... and so does Rena. Except that with Rena, there's no memories for Riku to experience as she doesn't have any; it's a void. Yes, it is essentially a mindrape. So after the kanshu is defeated, Rena is the one sobbing in a ball. CUE MASS AWKWARDNESS. This is the big lie, that Rena is a fully functioning human being with a complete set of memories. She's not, and it's this knowledge that makes her job really, really of the suck.
Rena's job
Let's be honest, being a jinki user epically sucks.
The first part is that they fight in secret. If an ordinary person were to get involved, they'd only get killed and it would be the jinki user's fault for getting them involved. Given that a jinki user generally fights to protect the ordinary people, this is a bad end. This is how Riku got involved in the first place and as a result of his involvement, he got given a doombaby and was turned into a kanshu himself for a volume.
The second thing is that they're dying from the same virus that creates kanshu in the first place. The only reason that they haven't died yet is because of the core in their hands that arrests the virus' spread. It's not a cure, it just allows them to stave off the eventual end. They're able to use the virus to help amplify their own abilities, but there's a limit to it and chances are the limit for how long they can hold their jinki weapons is based on how long they can use the virus without dying. When they do die, they'll turn into a monster that will slaughter everyone they've known and no-one will remember the person they were. To prevent this, their partner has to kill them before they make a complete transition (like, say, decapitation) and even then pretty much the only people who will remember them are fellow jinki users.
There's a wall in their headquarters filled with photographs, and Rena's wall in her house is filled with photographs. It seems very nice and cheery until you find out that photographs, unlike memories, continue to reflect the person who died. In other words, the wall of photographs is a cemetary -- and most of the people in the photographs are still alive! It's a very, very grim and sad thing.
Finally, if having to kill twisted animated corpses of people wasn't bad enough, there's a group of people who hunt down jinki users and slaughter them. We don't yet know what their motives are, but at least one of them, Takao, is working for the source of infection and is collecting cores for some reason. We don't know why, but Takao's easily good enough to take down a building full of trained fighters.
The thing is, this is the only life that Rena remembers. She doesn't have a family and she lives wherever she's told to by headquarters. Her entire life is based around this very grim outlook on life, where it is very likely that she will die tomorrow and no-one will remember her and so it is no real wonder that she is messed up.
The three faces of Rena
Rena has three distinct layers to her.
The first is the one that she shows in camp the vast majority of the time and the one that she shows at school. She's coldly aloof, dismissive of everyone and pretty much wears a :| face all the time. This is an act, and is consciously designed to make sure that no-one knows her at all. She doesn't want people to know her because to know her is to be brought into her fight and then die.
The second is the one that we see for most of the canon. Rena as a jinki user is a pretty classical tsundere. She is stubborn, blunt and a bit of a spitfire. She yells, she's proactive, she's a more dynamic person in this mode. She takes charge and expects people to keep up with her. She's all :| to everyone, with occasional blushy or flaily moments.
The final side is what I suspect to be the true Arisugawa Rena. This Rena is the one who puts photographs of her teammates so that if she becomes a lost rebound again, she'll have some record that her relationships ever existed. She loathes herself because of what she is and how dishonest she is with everyone. She's a deeply tragic and miserable figure, mostly because she's not a complete person. She never laughs. She never smiles. She's never anything other than mission focused or miserable.
The worst part is that it's a trap of her own making. She lies so well to people about how she's coping that they never look any deeper and so never realise that she's really not. The only person who's managed to get through all this to some extent is Riku, who's worked out that Rena is always sad, even when she doesn't look like it. I mentioned before that World Embryo is a story about lies and lying, and it is. Both Riku and Rena are liars. Rena's lies, however, are about ensuring that she disappears.
Given that this is a Moriyama Daisuke manga, I completely expect Rena to self-destruct at one point. After all, Chrno went comatose for a while, and it's kind of hilarious some of the parallels between them. I'm not sure when, given that the last scanlated chapter was for either February or March (FOR ALL I KNOW, IT'S HAPPENED IN THAT CRAZY MOONSPEAK LAND.). But I do think it's coming, and relatively soon. I'm not sure how I'm going to play it, but there is one thing I hope for. No aliens. :(