赵 is the first surname listed in the Hundred Family Surnames (百家姓), a classic text from the Song dynasty. It was placed first because the Song emperor's surname was 赵. Famous Zhaos include 赵云 (Zhao Yun, Three Kingdoms general) and 赵薇 (Zhao Wei, actress).
走 is the indexing radical, wrapping around the bottom-left - a "fast walker" motif you see in 趋, 起, 越. In 赵 the etymological link is faint; 走 was inherited from traditional 趙 where it framed the original phonetic 肖.
乂 in simplified 赵 is a 1956-reform placeholder that replaced traditional 趙's 肖 - an abstract two-stroke crossing kept the silhouette without preserving the original phonetic. Pure orthographic shorthand, no sound or meaning of its own here.
