The Mentor
There is a character, perhaps even a literal human, who McCrae speaks of as a teacher, a master, and a mentor. This character dies, and many of the poems are about the mourning process of the poet learning to cope with the reality that his favorite human is dead.
The fallen ones
The occasion for much of this poetry was warfare, so there are casts of people who are dead. The poems regarding the dead often encourage them to rest in peace.
Jesus Christ
Jesus occurs as a frequent character in the poetry, sometimes as a hero, sometimes as a challenging person. Jesus represents God to the speaker.
Death
Death is personified in the poetry. Once, death is the captain of a ship. Once, he is a harvester.