先代
せんだい · sendai · JLPT N1 · noun
predecessor
先 — 𠂉 (person variant) on top + 儿 (legs) below
先 originally depicted a person with a foot or step shown above the legs 儿, suggesting someone walking ahead. From this image of going before others came the meanings 'first,' 'earlier,' and 'ahead.' The modern shape preserves the 'legs' radical underneath.
代 — person radical 亻 + 弋 (stake/dart, phonetic)
代 is a semantic-phonetic compound: 亻 (person) gives the meaning and 弋 supplies the sound. The original sense was one person taking the place of another, which broadened to 'substitute,' 'replace,' and then 'generation' or 'era' — successive groups of people replacing the previous.