By request, I've added the RR6 bosses to the Favorite Boss Fight question. I've also added Dimension Shredder Yi Sang ahead of time to the Favorite Identity questions. As before, you can update your responses if you want to change your votes.
GregorHeishou Pack - Si Branch
Use his S3 or defense on turn 1 and every 3 turns after to maintain 
Serpent Arm giving him a passive 
Poise generation per turn and an additional Poise Potency generation when using skills against enemies with 
Rupture. It also buffs his S1 and S3. His S2 can give him an additional turn of the buff on crit with the last coin.
His S2 consumes 10 Poise Potency to give him a Coin reuse at 30+ Potency.
Serpent Arm gives him additional Rupture application on his S1 and Coin Power and crit damage on his S3.
Both his S2 and S3 inflict 
Deathrite【Venom】 which applies 
Offense Level Down and 
Defense Level Down on the enemy whenever their Rupture stack is triggered by a crit up to 5 times. When the debuff is fully consumed, it deals additional 
damage based on the enemy's Rupture Potency. His S3 consumes stacks of it before applying it again, fully consuming it if it crits and only partially consuming it otherwise.
His passive increases his damage dealt based on his Serpent Arm when his target has Deathrite【Venom】 and gives him 
Pierce DMG Up if he fully consumes their Deathrite【Venom】.
Combos with [Hong Lu] The Lord of Hongyuan's S3. The buff gives him 1 Poise Count whenever he triggers Rupture (1 per turn).
3 +4
62 (+2)Clash: 3-15Damage: 7.2-38.88Atk Weight:Poise on self + Rupture on target) (max 2)
[On Use] Gain +2 Poise Count
RuptureRuptureSerpent Arm, inflict +1 Rupture Count
4 +463 (+3)Clash: 5-23Damage: 19.2-123.12Atk Weight:Poise on self + Rupture on target) (max 2)
[Clash Win] Inflict +2 Rupture Count
[Clash Win] Gain +2 Poise CountRuptureRuptureRupturePoise, consume 10 Poise on self to Reuse Coin once (1 time max)Deathrite【Venom】Serpent Arm
8 +1464 (+4)Clash: 9-29Damage: 38.4-146.16Atk Weight:Poise on self + Rupture on target) (max 4)
- If this unit has Serpent Arm, Coin Power +1 for every 5 (Poise on self + Rupture on target) instead (max 6)
[On Use] Gain 3 Serpent Arm
[Clash Win] If target has 5+ Rupture, gain 5 PoisePoise on self + Rupture on target) (max 200%)Serpent Arm, deal +100% Damage on Critical HitDeathrite【Venom】 on target by 2Deathrite【Venom】 on target by 3Rupture CountDeathrite【Venom】
3 +10
60 (+0)Clash: 3-14Damage: 0-0Atk Weight:
Clashable Guard
[Combat Start] Gain 3 Serpent Arm next turn
[On Use] At 10+ Poise Count, Coin Power +1
Unbreakable Coin x5Deathrite【Venom】, deal +5% damage for every Serpent Arm on self (max 15%)
If this unit reduced the target's Deathrite【Venom】 down to 0 Stack, gain 1 Pierce DMG Up next turn (max 2) x4Poise to the Substituting Identity of the earliest Deployment order