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 द्वारा 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 आप would keep yur hate टिप्पणियाँ (if any hate comments, tho i cant see how आप could have any hate टिप्पणियाँ with something like this) to your own thoughts. thank आप very much!)