Lycidas is a poem by John Milton, often described as a pastoral elegy or an epic poem. Like most of Milton's writings, the poem uses religious imagery and characters to make a broader point. In this answer, we'll touch on some of the characters in the poem and explain how they fit into the broader theme of the poem.

John Milton

Milton himself is in the poem, as is his friend Edward King, portrayed as shepherds. King is actually Lycidas, and the poem is about morning and grief. We can see this poem as the way Milton grieved and coped with the loss of his friend.

St. Peter

St. Peter is also in the poem. In Christian theology, Peter is the gatekeeper of Heaven, so his role infuses Christian elements into this poem. Peter is used to show how material wealth and possessions can corrupt a person's life. Likewise, the personification of the river Cam, named Camus, portrays the grief Milton is feeling. In this way, both mythology and theology are used in the poem, broadening the appeal of the poem.

The characters in Lycidas are used to tell the story of grief and pain. The poem is written in a way that has universal appeal to the reader, for three reasons. First, grief and pain are universal feelings for the poor, rich, and everyone in between. Second, most people, even non-Christians, understand who St. Peter is and his role as the gatekeeper of Heaven, even if they don't believe in Heaven themselves. Finally, the use of mythology and shepherd imagery adds another element, as most people have an understanding of shepherding and mythology, especially people during the time this poem was written in the 1600s, when agriculture was still a very prominent part of the economy and people's lives. These are the main characters and themes in Lycidas.