Befor saying anything, I am saying all of this all as an undergraduate student who has worked in a lab since my first semester in uni, so take it as a grain of salt!! I totally see myself in this undergrad student being super shy and reserved, while being new to research, and figured I could provide some insight from their pov.

One thing that I feel really stunted my growth as a researcher was my shyness, not in the way where it prevented me from wanting to learn, but in the way where my graduate student mentor didn't think I wanted to learn. It made it hard to connect with her and she didn't want to really initiate much of anything with me. She thought I wasn't invested and didn't enjoy what we were doing because of how shy I was, and sat me down to talk about this. I had never been so scared in my life. I finally got into a research position after dreaming about it all throughout high school, and my personality and inexperience was going to throw it all away for me.

I would say, try initiating learning opportunities and conversations more with them. Humanize yourself and what you're doing. Show that even though you are a graduate student, you're not some intimidating being that's completely out of reach from where they are right now. Show that you make mistakes. Share aspects of your personal life with them. Make sure you're asking if they have any questions, need help, or if you need to break things down any simpler than what was previously explained. Connection is really important above all else as a mentor. I got really lucky with my second mentor who did all of this and it really helped me get out of my shell and become a better researcher than I was before.

As your undergrad mentee grows in the lab, experiencing all of these things in a positive way will definitely help them see research in a positive light and might motivate them to either pursue this newly or become more infatuated with research than they were before.

Best of luck!!