That's because there is a misconception regarding what introvert means. Most people think being an introvert is a synonym to being shy and that's not the case. You'll definitely see a much bigger amount of introverts suffering from shyness, but that can also affect several extroverts. It would take a very very long post to explain this here, but has someone who studied this, Lena actually displays almost all the traits of an introvert, from her way of thinking, to her problem solving, the way she relates with people and her base interests in the beggining of the game. It's actually quite consistent.
An example of this is how Lena tends to deal with her problems. When she's faced with a problem she first tries to think on things on herself and analyze everything on her own and then possibly come up with a solution. An extrovert usually needs to externalize this by confiding on someone else. It has a lot to do with how one's brain is more confortable dealing with this kind of situation. Some people need to be talking to someone else in order for their brain to make the necessary connections, while others need to isolate themselfs because otherwise everything else seems like noise. Obviously you need to take into consideration that this isn't a black or white thing and that sometimes even an introvert needs to confide on someone else to be able to process the information and sometimes an extrovert needs some alone time to do the same. But it usually has to do with how one feels more confortable in dealing with this type of situation on a regular basis.
Taking a look at the other girls, we haven't seen much of Cherry, but she also seems to display more introvert traits then extrovert and Holly is clearly an extreme example of an introvert. Cindy, Ivy and Emma are clearly extroverts. Alison and Louise are a bit on the middlegroung, but I'd say Louise pends to the introvert side while Alison to the extrovert, but as the expression goes, I wouldn't put my money on it.
Just one last note regarding your comment on how Lena chatted up Ian. My best friend is an introvert girl and quite shy, however, we met over 15 years ago and she was the one who actually chatted up with me and it was a quite similar situation to this one. I was in a coffee with a group of people I knew and she was there as well with some mutual acquaintances. I had a book with me laying on the table and after a while she started chatting up with me regarding the said book. Basicly she identified a common interest, made an evaluation/judgement towards me as a "non-threatening" figure and therefore felt confortable enough to start an interaction with someone that for her was a stranger. In this case there was also the factor that I knew some people that she knew as well and that facilitated the opportunity. In the case of Ian and Lena the opportunity came from Lena working there and attending to his table, which eased any sense awkwardness that would come from her to approach someone out of the blue.
