The abuse Kousei received at the hands of his mother is definitely sad, but the effect it had on the rest of his life is even worse. Of particular note is his internalized guilt, as he was fully convinced that she'd get better so long as he played better.
Kousei: Hiroko... do you think it reached her? I played the song as well as I could. (Breaking down) Do you think my mom could hear it? Hiroko: (Gives Kousei a Cooldown Hug while tearing up herself) Dummy - you just told me you're connected, right? You can bet your ass she did.
The Perspective Flip of the episode shown Saki was genuinely remorseful and aware of her atrocious behaviour, but she was terrified Kousei wouldn't be able to gain a living with her gone and desperately tried to perfect his talent. She dies alone, convinced her son despises her, hoping he will be able to thrive and be happy.
Hiroko: You call yourself a mother??? Kousei was doing his best just to make you happy-
Saki: I KNOW THAT! OF COURSE I KNOW THAT! *falls down and sobs* I do know that...but...I'm...running out of time...
There's also the added layer that this tragedy happened all because Hiroko thought that having Saki teach him the piano was a good idea. Imagine seeing your friend becoming very sick and pushing her son to the extent that he wished her to die... and knowing that it was all your fault.
Kaori slowly succumbing to her illness. Up until now, she's been able to keep a smile on her face and laugh it off in order to not worry her friends and, more importantly, Kousei but at the end of Episode 15, Kousei's just resolved to visit Kaori in the hospital after a lighthearted conversation with her about pastries and him promising to stop avoiding her and just visit her. The last two minutes of the episode has Kaori happily walking down the hospital hallway after the phone call when suddenly her legs give out on her. The look of distress as she tries to drag herself to the rail to pick herself up is shocking, but what's absolutely heartbreaking is that the viewer finally sees a glimpse of Kaori's frustration over her illness as she beats up her legs and yells at them to help her stand up and the last shot of the episode is her screaming in the dark hallway with her head in her hands.
On a similar note, in Episode 14, Kousei goes to visit Kaori after she didn't show up for their gala concert and as he's speaking, he is absolutely frightened at having to go through this again as he did with his mother. He's scared of watching Kaori get sicker and sicker just like his mother and eventually as seen in Episode 15, it takes its toll on him since that visit, he hadn't been to see her. It takes an angry phone call from Kaori and a promise to get him to finally decide to see her.
Tsubaki's inner conflict over coming to terms with her feelings for Kousei and how she wants to stay by his side, but she feels like music is taking him away from her.
After spent the day together, Kousei takes Kaori home on the bike, under a night sky full of stars. Kousei looks up at a shooting star and realizes Kaori is crying, but decides not to ask her why. She was crying because she knows she doesn't have much time left with Kousei.
Kousei finally coming to terms with his feelings for Kaori and deciding to visit her at the hospital with Watari only to arrive as something happens to Kaori and all they (along with the audience) can see is her condition worsen as she is having a convulsion and in the end, her hand weakly falls to her side.
Directly afterwards, as he's walking home from the hospital, the neighborhood cat he and Kaori would take care of gets fatally run over. This was especially horrible, as it was hinted but not explicitly stated that the cat was Chelsea, his childhood cat his mother threw out.
Kousei reverting to the state he was at when his mother died two years ago, this time as a result of watching Kaori get sicker and sicker.
Though it's not explicitly shown, it's suggested that Kaori passed away during her surgery while Kousei is doing his performance.
How the audience (and Kousei) becomes sure that was happened? At the middle of his performance, he enters into a somewhat dream-like state, surrounded by color and light. Suddenly, Kaori and her violin appears beside him. Kousei understands instantly that this means that the surgery was unsuccessful, but returns a smile at her and continues his performance as the vision of Kaori plays along with him. After a while, she stopped her performance, and the scenery slowly turns into night. Realizing that her time is over, he begs her to stay with him, but she disappears with one last tearful smile. In reality, Kousei completes his performance with tears running down his face, saying his final farewell to her.
Kousei: Wait... Please don't leave. I'll buy you more caneles. I'll call you on the phone for fun. I don't mind just being Friend A. Just don't leave. Don't leave. Don't leave me! Don't leave me without you...!
The letter that Kaori wrote for Kousei that her parents gave him when he visited her grave.
Kaori: "Dear Kousei Arima, it feels weird to be writing a letter to someone who was just with me a moment ago... You're an awful person. Laggard. Slowpoke. Dummy. The first time I ever saw you when I was 5 years old... It was during the recital for the piano school I was going to back then. The boy who appeared so awkwardly drove the audience into laughter when he bumped his butt on a stool. He sat before the piano that was too big for him, and the moment he played that first note... he became the object of my admiration. Playing notes as colorful as a 24-color palette, the melody began to dance. I was totally surprised when the girl next to me burst out crying. And yet, you're going to quit playing the piano. After influencing my life the way you did, how awful is that? (Cut to scene where Kaori as a kid, telling her parents shes giving up piano for violin because she wants Kousei to play the piano for her.) Awful! Slowpoke! Dummy! When I found out we were in the same middle school, I was elated. But how could I find a way to talk to you? Maybe Id hang out and go buy sandwich at the lunch concession? But in the end, all I could do was watch you. After all, you guys were all too tight (referring to Kousei, Watari, and Tsubaki's friendship), there was no space for me to slip into. I had a surgery as a child, and then got treated regularly as an outpatient. After I collapsed in the first year of middle school, I was in and out of the hospital, repeatedly. I started spending more time in than out. I was hardly in school by then, I knew that I wasn't very well. One night... when I saw my mother and father crying in the hospital waiting room, I realized that I didn't have much time. That was the moment, that I, started to run. I started doing whatever I wanted, so I wouldn't bring my regrets with me into heaven: I started wearing those contact lenses that scared the heck of me, eating a whole cake which I'd never done before because I worried about my weight, even playing the music scores that bossed me around so haughtily in my own way. And then... I just told a single lie: "Kaori Miyazono likes Ryota Watari"... was a lie I told. That lie would bring into me... Kousei Arima, that lie brought you into me. Please tell Watari that I'm sorry... Well, but still, I guess it won't take a long time for Watari to forget all about me. As a friend, he's a fun guy, but I'd rather have someone more wholehearted and earnest than him. Also... please also tell Tsubaki that I'm sorry. I'm just someone who's passing through, who'll be gone. I didn't want to leave an awkward mess behind, so I couldn't ask Tsubaki. Or rather, even if I asked her straight-up, "Can you introduce me to Arima?", I don't think she would've been okay with that... after all, Tsubaki was so in love with you. We'd all known that for a long time, and the only ones who didn't know that were you... and Tsubaki herself. The "you" that my underhanded lie brought to me wasn't really who I'd imagined. You were more negative and passive than I thought, not to mention stubborn, relentless, and a camera voyeur. Your voice was lower than I thought, and you were more manly than I thought. But you were... just as gentle as I thought. Remember when we jumped off the Courage Bridge? The water was so cold and felt so good, wasn't it? The round moon peeking into the music room, really looked like a delicious manju bun... When we raced that train, I really thought we could win! It was fun singing Twinkle Twinkle Little Star together under the shining star, wasn't it? There's just something about school at night, right? Don't you think snow looks just like cherry blossoms petals? As a musician, to be consumed by everything while off-stage... that just don't make sense, does it? Isn't it funny how the most unforgettable scenes can be so trivial? What do you think? Was I able to live inside someone's heart? Was I able to live inside your heart? I wonder if you'll still remember me at least a little. If you forget me, I'll just come back and... No, I don't want to start over. Please don't forget me. Promise me you won't forget me. I'm glad it was you... Will I reach you? I hope I can reach you... Kousei Arima... I love you. I love you. I love you. I'm sorry I couldn't finish all those canelés. I'm sorry I hit you so much. I'm sorry I was such a selfish brat. A million times, I'm so sorry. Thank you for everything. P.S. I'm enclosing something that I've always treasured, If you don't want it, feel free to tear it up and throw it away."
In the above letter, Kaori thinks that Watari has forgotten about her. In fact, he still keeps a picture of him with her on his cell phone.
What's the thing that enclosed in the letter? An old photo from before Kousei gave his first performance, featuring Kaori and her friend, with Kousei walking by behind them, making it the only photo Kaori has with him. Here it is◊.
Another black cat appears at the railroad tracks in the ending and disappears behind a passing commuter train, symbolizing the loss of Kaori.
The Downer Ending. More specifically, the last lines of this series:
Kousei: "Spring is coming up soon. The spring that I met you has arrived. What's to come is a spring without you."
The credits for the last episode. A picture montage/memorial for Kaori, accompanied by the more tearjerking acoustic version of the second ending song.
The first ending song, Kirameki may sound heartwarming, but closer inspection of the lyrics reveal these lines:
You're no longer here by my side, I'm well aware of that. Even then, I glanced back once more, thinking that you called out to me. And the sakura flowers at the arch we used to frequent, makes me feel like we could return to the past, even though they're now tinted in orange. And the times we spent together, are all irreplaceable in my heart. I'll treasure these memories you gave me, so thank you, and good bye.