How to justify research assistant's role in the grant application
The funders are more interested in the research question(s) you intend to pursue and your likelihood of successfully completing the research than they are about who, specifically, does what. Normally the PI has a guiding and supervising role over all team members and is the one to make critical decisions about the direction that the team takes. There is nothing unusual about that. Also, it is understood that some types of research require more than one person to be able to carry out the research in a timely manner.
I suggest that your application focus on the research questions and the need for a small team to be able to do the work in a timely fashion.
The supervisory role, properly construed, is appropriate for the PI. You guide and make important decisions. You provide the concepts/ideas that drive the research. Others help you realize those ideas and explore them. It shouldn't be a problem unless you imply that you wouldn't be &quot;involved&quot;.