ト, いたずら, あだ, かち
junior, emptiness, vanity, futility, uselessness, ephemeral thing, gang, set, party, people
JLPT3 G4 S10 F817
As per 土# (pile of earth) + 辵 movement (with its elements repositioned as in 從) → walk barefoot over uneven ground → *follower*; *foot soldier*; *fellow* (← follower/soldier etc. who goes barefoot, or on foot) → *useless* (← useless follower etc.) → *in vain*. There is no etymological connection between 徒 and 走.
[Full Kanji Etymology List]