not a teacher
Can I use "Hi dear" in message to a friend, if I am female and he is male? The message is "Hi dear, I am a little bit tired". Is it very intimate?
"Hi dear" is typically said by a husband or wife to his/her spouse, or a parent or other adult to a child. Unless you and your male friend have an established, close relationship in which you already call each other "dear", I would avoid using it.