Reinard-fox, this a good topic here
Eri : For her, he was jus a boy, a delinquent, in short, he was not interesting...
But behind the delinquent, there is a nice person ( Harima helps Eri... in many Eri's problem, there is Harima).
She also discovers a powerful man, she admires him and she falls in love.
She is the character who changes completely during the story (curiously, Harima change a lot too), of course the others change (Execpt Tenma who doesn't really change...)
There is too many sign during the manga (for instance when Harima was in the sinner's boat, he was holding a photo of Tenma... with Eri)
Yakumo : she can't read his mind, she finds something interesting : Harima likes the animals.
She discovers someone who needs help and she always wants to aid him. She shares a secret with him... but she can't declare her love, because she knows Harima loves Tenma.
Actually, she knows him very well, Tenma and Eri don't know him as well...