Assuming we ONLY get that person and none of their resources/allies AND they absolutely HAVE to protect you (ie, Cinder won't just leave you for dead if Ruby showed up or Emerald won't betray you if Cinder says so)...

Neo - Not only is she good at supporting people, she is a master at hand to hand, able to stand up to Maidens and was able to like 1v5 against a bunch of people who specifically trained to get good at hand to hand as well as having the advantage of knowing exactly what she could do. So she can get you out of danger and is impressive with hand to hand. She would serve as my closest bodyguard.

Emerald - The ultimate support and a decent fighter. The ability to cast such convincing illusions is so broken as an support utility if used right. We see how good it can be against a Maiden offensively and Emerald wasn't at ALL at the level she is now. Also, her moment in volume 5 with the Salem illusion. Her support ability is top tier and she would serve as support in the backline, enhancing the others, making my enemies fight worse like she did in Volume 3, and making it really hard to track my location as she could make multiple of me or make me invisible.

Cinder - Pure power AND a good fighter. She's shown to be on par with Raven who is considered one of the best fighters and can defeat Neo in hand to hand 1v1. Her power is obviously because of her Maiden power. Just a pure destructive force. If one and only goal is to protect me and nothing else, it's a no brainer for me, especially with Emerald augmenting her.