Who is Septimus?

Septimus is a Hollower working for Emrys’ father, Endymion and looking for the Ring of Dispel. Like others at the Guild, Septimus bullies both Tamsin and Cabell while relying on them to find information he wants. After forcing them and Neve to go to Avalon, Septimus constantly puts them ahead of himself and even attempts to feed Tamsin to the Children to save his own life. This fails when Tamsin stabs him, leaving Septimus to die instead. He is later reborn as one of the Children, and it is heavily implied that he has some awareness of who she is.