Did Kaori know that Arima loved her? From the letter she gave to Arima, she confessed everything and even gave that photograph, but at the same time, she told him to throw it away if he wanted to. This according to me suggests that she didn't know if Arima loved her. If she knew about Arima's feelings, she wouldn't have said it to him. Is there any strong evidence whether Kaori knew about Arima's feelings in the Manga/Anime?
I've only watched the anime (and a few months ago at that) but truth be told, I'm of the opinion that she did know that Arima had feelings for her. Take what I'm about to say with a grain of salt though as I do think that Kaori's true goal at the end was to get Arima to love himself somehow by whatever means she could. I don't think her true intention really was to get him to fall in love with her either but become a part of his life that would would galvanize his restoration.
Here are my thoughts:
Throughout the series, Kaori shows immense respect towards Arima's skills having admitted at some point later in the series that he was a source of inspiration for her. As a result she seems overwhelmingly distraught at the state of disrepair that Arima has fallen into. She initially enters his life by the first performances together and using Watari.
As this goes on its easy to see by everyone in the series that Arima has formed a dependency towards her (especially Tsubaki), and whether or not these are romantic, Kaori seems to realize this or even if she doesn't the scale of meddling and pushing Arima to strive towards reclaiming some former glory increases. Unfortunately Kaori is doomed, and then realizes that all her effort to push him to revitalize him is at stake and gives him her final wishes etc in an effort to convince him to move on with his life after her. I think this acknowledgement of dependency and her trying to help him move on from her as a result is key to the idea that she knows he loves her.
I know I'm making Kaori seem manipulative (or unknowingly manipulative) and maybe I'm putting too much confidence in what she's capable of doing but at the least that's how I interpret it. As with the shows name, she lied. Kaori told Aroma, Watari, Tsubaki, and everyone else, that she loved Watari. By telling everyone this it was known as a fact. But then you realize she loves Arima instead. Resulting in," Your Lie in April," because she lied in the month of April by using Watari.
This question eventually led me to a translation of the letter she had written for Arima.
Roughly translated, the ending part of the letter gives us an insight in what she knew and what she thought about Arima his feelings. She thought that he loved Tsubaki, or rather everyone knew that but apart from those 2. She, however, did love Arima. She factually said that at the end of the letter multiple times and asked him to remember her... if the letter reaches him. The way it was written indicates that she is "aware" about him loving someone else which means that he doesn't love her as a lover, in her eyes, but probably as a friend or musical partner.
Whether this thought from her is right or wrong is something I can't validate. If anything, I'd say that he loves her more than Tsubaki, but Tsubaki was most likely his first love though.
But to answer your question, according to the letter written by her, she was unaware about his feelings for her. As she wrote "I wonder if I made it into yours".
(It's been a while since I watched this one, watched it when it was airing)
Sources: finishing the anime and of course, the translated version of the letter.
They both knew they liked each other but didn't express it verbally.
Did Kaori know that Arima liked her? Yes she did. At first while attempting to make a lie like that after understanding she doesn't have much time. She didn't know that it would reach him or she can get close to him or here the main question. If he likes her or Tsubaki but still she went on ahead and said it to tsubaki to introduce her with Watari so she can get more closer to Arima.
With time we see that Arima was wanting to be with her more and then she herself had to tell Arima to go to Tsubaki that scene where he carries her. Arima saw Tsubaki just as a friend. But the moment she actually understood that he loves her is when she asked during that firefly scene and also when Arima took the initiative took the initiate to play the piano even while she didn't appear at the place. She later asked why he played and she thought he would just leave but he said to her that he knew she was with him all along.
But the main beautiful part about this anime, Even tho she knew he liked her, She didn't want him to confess because she knew she didn't have much time and she just wanted to live her life and make memories with him with the performances and most importantly she didn't want to get even closer so that he will get attached to her and then he will go back to a heavy depression so she didn't and kept talking to Watari which looked like a barrier between them to say the truth verbally and she kept interepting some of the conversation to talk to watari.
But the main reason and sole reason when she understood is when he tells her that she wants to play with her last time and since she liked him a lot too. She gave the surgery a try but it failed and hence the lie stayed a lie until she confesses how much he means to her and how much of an inspiration he was to her.
In the beginning she also didnt want to come btw Tsubaki and arima but later she understood he liked her more and with knowing that she also wanted him to be with her after she is gone thus not even mentioning this to her or breaking the friendship the 4 of them shared. Their hearts knew it but it was best it wasnt said verbally. That was the most beautiful part of this anime series. Trully an amazing one.
Towards the ending of the anime, we realize that Kaori's sole motivation in life was Kousei and the love she had for him. Everything she did, including using contacts, changing her hairstyle, "liking" Watari just so that she could get closer to Kousei, was all done in mind that she may not live to continue the ballad. Keeping all this in mind and noticing that she executed her plan quite successfully, we can conclude that Kaori was intelligent enough to know that Kousei loved her. Also there were moments when Kousei told her personally that he loved her, though worded a bit differently (like that scene where they saw the fireflies).
(I just finished watching the anime and the plot is freshly imprinted in my mind. This is my honest opinion and I am quite sure of it too)