Answer:
The companion who accompanies the speaker in the poem is his sister.
Step-by-step explanation:
"For thou art with me here upon the banks
. . .
May I behold in thee what I was once,
My dear, dear Sister!"
In William Wordsworth's poem "Lines Composed a Few Miles Above Tintern Abbey," the speaker is visiting a place he has seen once before. However, this time, he sees it in a different way. The speaker is now able to stop and appreciate the natural beauty before his eyes. This second visit has a more reflective nature, the speaker now not only taking in the beauty of the place but also developing deep and lofty thoughts about it. All the while, the one who is with him, as is shown in the lines above, is his sister.