Considered one of the most tragic and moving anime of its time, Your Lie in April has broken the hearts of music lovers and non-music lovers alike. Kaori and Kousei's whirlwind romance is a bittersweet tragedy that ends in loss. It also completely overshadows the other complicated dynamics within the main cast.
One of the characters that fans lament about most is Kousei's childhood friend, Tsubaki. She is a kind and athletic girl who literally carried Kousei on her back when he needed it most. Certain factions of the fandom have felt that her relationship with Kousei was far more realistic and relatable than the central romance, largely because of the surplus of time the two have spent together, especially when compared to the quick and stormy relationship that Kousei and Kaori shared.
Despite how much more history Tsubaki has with Kousei, she did not realize her feelings until after he had already fallen intensely in love with Kaori. Thus, he did not reciprocate Tsubaki's feelings in the anime. However, with Kaori's untimely passing, is there hope for Tsubaki after all?
In Episode 14, "Footsteps," Tsubaki was heartbroken after Kousei revealed his plans to leave town to pursue music. She also realized she feels lost without him, which ultimately led to her discovering that she is in love with him. This prompted her to dump her boyfriend in the following episode. Yet, like anyone who wants the best for the person they love, Tsubaki selflessly helped Kousei realize his feelings for Kaori. After she passed, Tsubaki promised Kousei that she would never leave him alone, acting as his guardian angel. Assuming she'd keep this promise, which seems likely given how averse she was to Kousei moving away, one can safely assume that Tsubaki would continue being the person nearest and dearest to Kousei.
It is also important to note that, despite how intense Kousei and Kaori's love became, they were in junior high school and they only knew each other for about a year. If there is one thing that can heal a brutal heartbreak, it is time. Even if Kousei never fully gets over Kaori, Tsubaki would be the one person who is most likely to understand his feelings. Furthermore, because her feelings have been fully realized, she would not be likely to forsake her position as the one who knows him best; the girl who's known him best since the beginning.
As enticing as fulfilling the childhood friends-to-lovers trope would be, these theories cannot be confirmed without a sequel. From the looks of it, there doesn't seem to be one on the horizon. Given how conclusively the series ended, it doesn't seem like fans would want a continuation anyway. Still, one can't help but root for Tsubaki, the underdog girl-next-door, and hope that she can help Kousei eventually find wholesome love again.