The returner himself is surely associated with the special magic. Since the title emphasizes that a returner's magic should be special, he is likely the main character with this unique magic.
There might be a mentor figure in the 'A Returner's Magic Should Be Special Manga Novel' who also has a connection to the special magic. This character could have either taught the returner how to use the magic, or they might share a similar type of magic. Their role could be crucial in helping the returner understand and master the special magic. For example, they could provide insights into the history and origin of the magic, which in turn helps the returner unlock its full potential.