Rachel's weakness is Dark magic, therefore, Sonic can erect a barrier around Rachel to protect her from the Dark magic in the atmosphere around them. (this cant, however, protect her from Dark energy directed at her によって an attack)
Reason #2:
The only thing that Sonic can't do, is swim, henceforth, Rachel, being part mermaid, can work with sonic when forced into an aquatic environment.
(once again, please, no hate comments, this is an "opinion article", so i would appreciate it if あなた would keep yur hate コメント (if any hate comments, tho i cant see how あなた could have any hate コメント with something like this) to your own thoughts. thank あなた very much!)