This book constitutes the proceedings of the First ULF-EnC 2025 Challenge on Enhancing Ultra-Low-Field MRI with Paired High-Field MRI Comparisons for Brain Imaging, held in conjunction with MICCAI 2025. The 19 papers included in this book were carefully reviewed and selected from 21 short papers submitted to the proceedings. The challenge addressed a timely and clinically significant problem: bridging the quality gap between ultra-low-field (64 mT) and high-field (3T) brain MRI through algorithmic enhancement using paired imaging data.