I'm the one who accompanies you all the time.（表达一种目前状态）我是一直陪着你的人.
I'm the one who will be accompanying you all the time.（将来）我是会一直陪伴你的人.
I'm the one who has been accompanying you all the time.（开始于过去并一直持续到现在）我是一直以来陪在你身边的人.