WHOLE CHARACTER
为
to do; to act; to behave (为人,为难); to serve as; to become (成为,当做); to count as; to be (十两为一斤); (written style) a passive or emphatic marker (为天下笑)
为 swaps roles: wéi to do or become (成为, 认为, 行为) and wèi for someone's sake (因为, 为了, 为什么).
ZDIC: 会意 — the old form pictured a hand leading an elephant (doing work). The modern four strokes are fused; taught as one written unit with no invented pieces.