Your faculty member you support is your biggest advocate for support in the course. Your faculty member can help you advertise your sessions, and provide insight on what students are struggling with the most.
Chat with your faculty regularly, provide them with info on how sessions are going
Ask your faculty for resources to share in sessions, like worksheets or practice exams
Ask your faculty for letters of recommendation, or references for other jobs
Meet with your faculty for anything outside of regular SI activities
Grade papers/homework/exams for your faculty
Teach for your faculty member if they aren't able to make it to class
Proctor exams for your faculty member
Your students will look up to you as an important source of knowledge and help for the course. You are a model student yourself as someone who has performed well in the course. Be a role model for your students, but make sure you walk the line between SI Leader and Friend carefully.
Bring a friendly presence and helpful attitude to best serve the needs of your students in sessions
Feel free to set-up a groupchat with your students to keep them up to date on when sessions will be held
Get to session early and chat with students about how their weekends were, or just generally catch up with them
Connect students to helpful campus resources such as the counseling center if they should need it
Give your phone number to students. Students will think they have a personal hotline to you and may constantly request your help outside of sessions and office time
Accept gifts from students. If a student offers you a gift (buy you lunch, a drink, etc.) this is a conflict of interest and may appear to students as if you can be bribed for your assistance. If they want to give you something after your last session is done, and you are no longer their SI Leader, that is fine
Get involved in a romantic relationship with a student during the semester. Like in the example above this creates a conflict of interest. When the students are no longer yours at the end of the semester, you are free to socialize with them in any way you choose
Put yourself in a compromising position with your students that is unbecoming of your position