She is a friend. You are not yet close friends, but you are friends : ) All friendships start small.

We just say "friend", and it can be used for either a male or female. "boyfriend" and "girlfriend" mean someone you are having a romantic relationship with. "acquaintance" is very formal and isn't used in everyday writing and speech, and it just means someone you know, not someone you do things with, and it also doesn't show whether you like the person or not . (that is why you didn't find it in your texts)

"Casual friend" I have many casual friends, people I see and talk to on a regular basis, we have a chat and get along together fine. I'd never refer to somebody who I studied with and often had a nice talk with as an acquaintance. To me an acquaintance is a friend of a friend of my sister in law's best friend.

A 'friend' does not have to be someone that you are close to. I would agree with merlyn that you could use it here, and it seems to me a better fit than acquaintance which is both a bit formal and also a bit impersonal. An acquaintance is more like someone you've met but never really spoken to properly. I've seen this observation before from speakers of other languages, that we use the word 'friend' too loosely in English. But we can qualify it to explain our relationship with them further; best friend, good friend, close friend, old friend, work friend, mutual friend, boyfriend, girlfriend and so on. As for 'mate', 'buddy' and 'pal', they are normally used to describe the relationship between males who are friends. In the UK, it is certainly odd to refer to a woman using any of those three words.

Hi Marina, I would say acquaintance. Or perhaps a study partner. Are you in the same class together? Because then you could also call her a classmate. You also don't necessarily have to use any certain word. If you're referring to her in conversation, you could just say, "A girl I know," or "A girl I study with."