Be in somebody's corner
Definition 1 of 1
Pro Tip 1/3Loyalty Connotation
The phrase implies strong, loyal support, like a coach in a boxer's corner during a match. 
When everyone else doubted me, my sister was firmly in my corner.
Pro Tip 2/3Emotional Support
It's often used to express emotional backing beyond just agreeing with someone's opinion. 
After the breakup, my friends took their place in my corner to help me recover.
Pro Tip 3/3Confidence Booster
Knowing someone is 'in your corner' can boost your confidence in facing challenges. 
His mentor's assurance that he was in his corner made him much more confident.