J1 - 40 (20/20) - huge combo starter, cancels into SJ and U, which is optimal on spaced hit as J2 won't connect
J2 - 40 (20/20) - cancels into WJ and SJ
J3 - 60 - cancels into dash, U, I and SI
WJ - 140 (20x7) - very fast huge multihit teleport that doesn't work on the corner, cancels into SJ, U (doesn't combo), WU (triggers hitbox) and WI
SJ - 120 - frame 1 invincible knockdown
KJ - 150 - safe disjointed knockdown poke
U - 120 - fast knockdown projectile and combo finisher
WU - 150 - knockdown counter with virtually no endlag when triggered
SU - 160 (80/80) - knockdown shieldbreaker
KU - 20x - descending multihit combo starter
I - 320 (40x8) - fullscreen multihit disjoint and combo starter
WI - 250 - vertical wall in front of Byakuya that can block projectiles
SI - 500 (25x20) - multihit lock-on that doesn't work on the corner
Safe moves - J1, J3, KJ, KU; WJ, SJ, U and WU can be safe in some scenarios
Whiff punishes - J, WJ, U, I
Dash coverage - J, WJ
Anti airs - WJ, SJ
Mixups - Byakuya Bankai can effectively either zone or zone break the vast majority of character, and some matchups require both, so it's good to know when to be aggressive with WJ and when to poke and zone with J, SJ and U
Combo routes - bnb - KU J1 J2 WJ WU/WI; juggle setup - after WJ you can follow their dash or cover no dash with J
Shieldstring finishers - J1 SJ, J2 WJ, J2 SJ, J3 L, WJ WU into anything
Zoning - J, KJ, U
Out of shield - SJ; WJ punishes laggy spaced moves
Zone breaking - WJ
Reversals - WJ punishes dash coverage attempts; SJ punishes okizeme attempts
Byakuya Bankai is a very complete character that doesn't really require specific assists, so they're mostly preference dependent