If you ve read it to the end, or read the comments here. Then you'd know its not a super happy ending.
Spoiler
He thought she died and wondered around everywhere except that island they lived in.
She survived and lived in that same island.
They lost communication and only found each other when they were at the end of their lives.
So yes, they end up together. But again their time together is only short left. They basically wasted all their lives apart and no communication. Then found each other when they like have 5 more years left. Lol.
People wish she either stayed dead. Or they just met sooner.

Why do people keep saying I hope they change the ending in the anime adaptation.. isn’t she alive?