Speaker
The speaker is the only character in the poem. He is the protagonist, the first-person narrator, and the focus of the poem, despite its title. He is looking at the titular tree while in a reflective and plaintive moment early in life. Despite expecting to live another fifty years, he feels the presence of death becoming imminent.
The speaker is twenty years old. He believes he will only live another fifty years. This means he expects only to live to age seventy. While this might seem unusually short compared to life expectancy today, at the time of composition it was about average.
Unexpectedly for one so young, the speaker does view another fifty years of existence as a great surplus. He already feels that it will still not be enough time to enjoy all the beauty and pleasures that life has to offer. The cherry tree becomes the symbol of everything that he wants time enough to enjoy.