In this case it's sort of a tough call. Unless this is her head on the first page over
here I have a hard time referring to Lanka as the "first girl" by any stretch of the term since the flashbacks later don't count, and Hitomi is the first candidate to interact with him from our perspective but there's clearly something lacking between them for romance to bloom at the moment.
If I had to say, I'd still put my money on Lanka but I would not even be a little surprised if this series just ended without an established coupling. There is sort of a harem vibe here (it even bagged itself the tag for that), most likely patterns would be Jin and Lanka or Jin doesn't decide like a little bitch and the story just fades out with a "But our story is just beginning... but you don't get to see anything else, sucks to be you"