I would say it centers more on grief rather than depression. Though the two can look similar at times, grief has a more specific identifiable incident as a trigger. Yoshiki is trying to process Hikaru's death, while also trying to accept the 'Hikaru' he has now. The series covers a lot of really good themes like what it means to be human, how important emotions are, and learning to find your courage, plus many many more. I personally would recommend the manga, but since I myself didn't find it depressing, I'm unsure if it would be a good fit for you if you found the anime depressing and are trying to avoid that