Watari is the playboy of the show. He is the best friend of Tsubaki and Kousei and he tags along with their development throughout the show. Though he ends up being used by Kaori to fulfill her greater ambition, he actually presents a very important message to the viewers. Many of the characters in the series have tragic backgrounds or harsh regrets.
Watari is one of the most normal ones.

And that’s exactly the point. Many a times, we imagine that we are the hero of the story and have to carry the greatest of burdens on our shoulders. But that’s not usually true. We are usually just the average Joe with very normal everyday problems. However, it doesn’t diminish his character in the slightest. He provides support to his friends, lives each day with enthusiasm and grieves when tragedy strikes.
Being normal is completely okay.

Many viewers won’t remember who Saitou is just from hearing his name alone. For all you viewers out there suffering from amnesia after a year of not watching the show, Saitou is the guy that Tsubaki dates during a majority of the series. She confesses to him and they start dating early on, developing a strong rapport.

However, Tsubaki is actually in love with Kousei but is unable to come to terms with it. Saitou is one of the first characters to learn the theme of the series; letting go. He recognises what Tsubaki doesn’t, and breaks up with her not out of malice, but out of care. Rather than drag on the relationship for the sake of attachment, he lets her go so that she can pursue the one that she can truly love.

Nagi is the younger sister of Takeshi Aiza, one of Kousei’s rivals. She initially despises Kousei because of her natural loyalty to her older brother. Later on, she gets saddled with Kousei as her tutor and is extremely unhappy about it. They have to practice a piece for her school performance and she constantly protests over that arrangement.

Over time, she gets closer to Kousei but is still nervous about performing for her schoolmates. This nervousness carries forward to even the point right before the performance. However, when it’s her time to shine, she buckles down and performs with all her heart. She presents a simple message of growth, and her natural character development shows us that anyone can become a better person each and every day.

Tsubaki has been best friends with Kousei since young. She has seen his rise and fall as a musician and has stayed with him every step of the way. She harbours a crush on him but doesn’t seem to be aware of the fact. Because of this, she is reluctant to “allow” Kousei to revisit his musical roots, because she fears that music will take him away from her.

The important fact here is that she goes along with the theme of the show despite her reservations. She let him go so that he can pursue his own happiness. Despite that, she still remains his friend and doesn’t angst about his decisions at all. Sometimes letting go isn’t just about our happiness, but that of our loved ones as well.

Like Kousei, Takeshi has been playing the piano since young and religiously practices each and every day. He was inspired early on by the methodical and masterful performances that Kousei puts on and he dreams of surpassing him as a result. This all comes crashing down when Kousei faces his tragedy and quits out of the blue.

While viewers and characters close to Kousei understand why this happened, Takeshi wouldn’t know why his inspiration has disappeared. Despite that, he continues to practice constantly to pursue the figure that he looked up to, even if it is now a ghost and no longer present. His dedication alone is a testament to what any person should aspire to. And now, years later, he gets to pit himself against Kousei at last.

Emi has similar roots to Takeshi with regards to Kousei. She sees a performance of his while she is still young and gets inspired by the high level of performance. However, here is where the deviation lies. Takeshi sees Kousei as a rival, but Emi sees him as an ideal. She saw how pure and passionate his first performance was and becomes very troubled when his later performances become dull and monotonous.

She spends a majority of the series trying to push Kousei further in order to once again see the Kousei that she believes is real. Her insight into his character seems to be very unique from the other characters. And she shows us that sometimes a different perspective is important when evaluating our lives.

Saki is vilified by the show early on and is often attributed as the reason that Kousei has lost the ability to hear the notes that he plays. Often, she is portrayed with a shadow cast over her face as she pushes him harder and harder, straying further into abuse territory the more she goes on. However, we later learn that she has good intentions for doing so.

Upon learning that her son has talents in playing piano, she pushes him to become the best he can be because she wants him to have the capability to make a living when she’s gone. This, coupled with her desperation brought on by her terminal illness causes her to act more and more deranged. While it might sound macabre, Saki shows us just what kind of lengths a mother will go to protect her children, even if it means to harm them as a result.

Hiroko is the tutor of Kousei and she was the one who initially introduced Kousei to the joy of playing the piano. When she discovers his natural talent, she encourages him and his mother to allow him to be tutored by her. This goes well at first. He advances at a significant rate and everyone is proud of what he has done. But it all goes into a downward spiral his mother’s death.

With him falling into depression and Saki departing the mortal realm, the one that gets saddled with all the guilt is her. She constantly second guesses her decision to bring him into the world of music. The inner turmoil that comes with her new responsibility for Kousei coupled with her guilt must be absolutely crushing on a normal person. Despite all that, she forges forward and does her best to take care of her daughter and Kousei at the same time.

It’s understandable for anyone to give up under these circumstances, but she didn’t. And it’s a simple concept, don’t give up.

Kousei is one of the protagonists of the series. When his mother left the mortal coil, his ability to hear the notes he plays goes along with her. For years, he has lived under the crushing pain of depression at his inability to pursue the art that he once held a great passion for. As the series goes on, he learns to acknowledge that it is not her death but his fear that holds him back.

And he faces his fear head on by the end, playing a beautiful song with the greatest passion he can muster. It is said that courage is not being fearless, but having fear and carrying on in spite of it. Kousei possesses a courage that very few possess. After all, who can claim to shine despite being under the oppressive weight of depression and the death of a loved one?

“We should live our lives to the fullest. You never know, we may be dead by tomorrow.”

Kaori is a violinist that was also inspired by Kousei when she was younger. Originally, she played the piano. But after seeing his performance, she dropped the piano altogether to play the violin because she dreamt of playing alongside him on the stage. However, she is hindered by a crippling illness that causes her to constantly revisit the hospital.

At some point, she learns that she might die anytime from her illness, despite her parents’ best effort to keep it from her. Yet, she doesn’t give up. Instead, she starts to live each day like it's her last, because it could literally be her last. She eats what she wants, dresses how she wants and does everything in her power to pursue Kousei despite being too shy to do so before.

We don’t need an illness to do the same. Each day could literally be our last. It is very easy for us to do due to a freak accident or a sudden onset of illness that we couldn’t diagnose earlier. So like a violinist once said:

“We should live our lives to the fullest. You never know, we may be dead by tomorrow.”

And so she did so. Starting by contacting Tsubaki and pretending to like Watari, all so she can get close to Kousei.

It was her lie in April.