When it comes to expressing the idea of supporting or backing someone, there are a variety of synonyms for the phrase "be at the back of". One alternative is "stand behind", which conveys the idea of offering support and standing by someone's side. Another option is "root for", which emphasizes being a strong advocate for someone or something. The phrase "have someone's back" also works well in this context, emphasizing loyalty and support. Furthermore, one can use "champion" or "defend" to indicate actively promoting or safeguarding someone's interests. These synonyms provide diverse ways to articulate the concept of being supportive and standing up for someone.