Random musing after the fact..
There are 7 (8 with patch 1.3) unique NPCs. 4 (5 with patch 1.3) female NPCs and 2 male NPCs. The 4 female NPCs are all Legendary (Gold) while 1 male NPC is Epic (Purple) and 1 Legendary.
There are 3 warrior, 1 brawler with 2 variations, 2 ranged, 1 marksman with 2 variations, 2 support, and 2 caster classes.
Warriors (Sword)
Shield Warrior - Raises Defense of Self up for 50% for 6 seconds
Sword Warrior - Attack down horizontal row
Axe Warrior - 100% crit rate for 10 seconds
Brawler (Glove)
Defensive Brawler - Shields and clears all debuff/DOTs on allies for 3 seconds
Offensive Brawler - Attack all enemies
Ranged (Bow)
Thief - Heavy attack to one enemy and causes bleed for 5 seconds
Archer - Massive attack to one enemy in back row
Marksman (Crosshair)
Rapid Fire Marksman - Increased attack speed for 8 seconds
Piercing Marksman - Attack down horizontal row
Support (Heart with Cross)
Healer - Heals lowest HP ally
Witch Doctor - Curses enemy and lowers their defense by 5% and forces all enemies to target them instead of your allies for 5 seconds
Caster (Wand)
Fire Mage - Rain Fire on all enemies and apply burning DOT
Ice Mage - Rain Ice on all enemies , lowering their attack power with a chance to stun for 1 second
Unique Females:
Pink Haired Swordswoman - Deal heavy damage down a row of enemies
Black Haired Gunner - Deal damage to all enemies and stun them for 1 second
Gold Haired Elf Archer - Deal heavy damage to all enemies
Silver Hair Witch - Deal heavy damage to all enemies every second for 5 seconds
Blue Priestess - Heals all allies for a large amount
Unique Males:
Swordsman from zone 1 - Deal damage down one row
Brawer from Zone 4 - Increase attack power of all allies for 7 seconds
MC - Cleave front row of enemies for massive damage with execute effect under half health
My party for the clear -
MC, 4 Unique Female NPCS, 1 Healer, 1 Brawler, 1 Witch Doctor, 1 Fire Mage
Healer and Brawler are a must for the last enemy and getting a nicely timed shield to block the whole party AOE damage is nice. Fire mage provides DOT and Witch Doctor debuffs the last enemy and CAN make it target itself for hillarious results. However, timing is an issue so you have to be lucky.
Equipment is somewhat cut-and-paste, but the substats are the important parts to look at.
There are 5 levels of quality to each piece.
White - No substats
Green - One substat with higher change of high/max rolls
Blue - Two substats with medium chance of high/max rolls
Purple - Three substats with higher average stats, but low chance to get max rolls
Orange - Four substats with higher average stats, but low chance to get max rolls
You can roll the same substat for stacking bonuses that are higher than lower quality which is why max rolls are rarer.
Example: Max Skill Cooldown is 20%. You can get a Green with 20% easily. A blue can get 12+15. A purple could get 9+10+12. An orange can get 12+13+11+10.
Of course you could win the lottery and get god rolls.
I found a Purple level 7 glove for my brawler that was 20+20+19 skill cooldown. My healer had to make do with a +20+19+11 wand from level 5.
Gear level just determines base stats, not bonus substats.
Level 1 - Forest Island City Shop
Level 2 - Forest Cave (Clear 50% of Lava Island)
Level 3 - Lava Island City Shop
Level 4 - Lava Cave (Clear 50% of Round Stone)
Level 5 - Round Stone City Shop
Level 6 - Biostation Cave (Clear 50% of Sand Island)
Level 7 - Sand Island City Shop
Level 8 - Sand Cave (Clear 50% of Dates of the Day)
Level 9 - Dates of the Day and Snow Island City Shops
Level 10 - Date-Collapsed (Clear 50% of Snow Island)
I'm guessing there is no Snow Cave because there is no Level 11 loot even though it's on the map and doesn't open when you retake Forest Island.
Oh yeah. Character rarity is a thing and determines the base stats and scaling of the character. Legendary characters have the highest scaling and base stats, but honestly the bonus from gear outweighs this benefit.
You can start to rarely see Legendary/Gold characters from Sand Island onwards. With the odds being Sand Island < Dates of the Day < Snow Island
You have a higher chance of getting a POW of a higher rareity from the caves, but its still random.