I think Kwai Lan(Rain) will end up with Bad Guy Kin(Kenneth). Here's my reasons:
1. Kwai Lan was always shown the good side of Bad Guy Kin. First by his gramdma's account, then she saw that side with her own eyes when she accompanied Bad Guy Kin to take his grandma to the doctor. Bad Guy Kin even thanked her when he asked her to take care of his grandma. These scenes were shown for some reasons. I think Kwai Lan will gradually grow fond of Bad Guy Kin.
2. Kwai Lan already know Siu Ting(Cerina) loves Fei Hung(Bosco). Being a good 'sister', she won't 'steal' Fei Hung away from Kwai Lan. So, this mean if someone is interested in her (e.g. Bad Guy Kin) she might accept if she has a good impression/feeling about him.
3. The scriptwriter must leave Fei Hung single for 13th Aunt.
This is all just my speculation. It might not be true at all. I am only up to episode 6 (or 8, I forgot).
I decide to watch this series solely because of Bosco Wong, but I end up liking Bad Guy Kin more. His ambiguous personality is very interesting. I especially enjoy seeing his scenes with his grandma and Kwai Lai.
I found the character of Wong Fei Hung very contradictory. On 1 hand, he is very immature and naive and rather childlike. He gets into trouble often because of his rash personality, and he seems to not know the many of vices in his surrounding world. On the other hand, he sometimes also act very maturely, like when he knows Leung Foon (Sammul) is secretly learning his kungfu, and he just let him, because he thinks it's better for Leung Foon to secretly spy on him that go get into trouble somewhere else. And when Fei Hung lecture someone (a thing he does very often), he sounds so mature.
Even though I like Bosco, I must say that this contradictory personality might most probably be due to Bosco's acting. Bosco said he studied the Kwan Tuck Hing and Jet Li potrayal of Wong Fei Hung because he wants his portrayal of the younger Wong Fei Hung to be a natural progression to the character played by Li and Kwan. He might be trying too hard to follow their potrayal but the scriptwriter might not have written the script with Jet and Kwan's Wong Fei Hung in mind which resulted in the character become contradictory. Well, that my opinion about it.