If someone's older than you, you can't use 너 talking to them even if they're your extremely close friends. I don't think there're any variations of "You are my friend" 제 is a humble word for "my", combines with "가" like "제가 할게요".

You are my friend=너는 내 친구야, 넌 내 친구야

The right way is 너는 내 친구 or you say it by using name e.g. Destiny 는 재 친구

I think also, it depends on the context of why you are saying "You are my friend". e.g. Friend: "What did you say to me?" You: 넌 내 친구 Compare this to, Friend: "Who is my friend?" You: 넌 내 친구 I think 넌 would be wrong in this situation. [넌 is short for 너는] But I think in speech this is omitted (usually) and you would just say 너 내 친구. Also, I think if you use 재, then you should use 씨 honorific. --------- thanks Hailey! ^^