From Wildermyth Wiki
Example upbringing showing the generated backstory, hooks, and bonus stats

Upbringing is a permanent, unchangeable attribute that is randomly generated for every hero. The upbringing is responsible for defining permanent stat bonuses, the hero's hooks, and the automatically generated backstory. This system is intended to make every encountered hero unique.

On a technical level, the upbringing consists of an origin, an anecdote ("dote" for short), and a motivation ("mote" for short). Each of those parts comes with a defined hook, history tags (defining aspects of the character's backstory, e.g. eccentric, criminal, happy childhood), personality stats (e.g. leader, flirt, hothead), and combat- and campaign-relevant stats (e.g. health, melee accuracy, charisma). Heroes that are generated as children (joining the party during an interval as they reach adventuring age) have their own separate set of origins, meaning that the probabilities for obtaining certain hooks or desired stat combinations is different. Also, a bonus for armor, stunt chance, and/or bonus damage can only be obtained with such a child origin. The game will not allow combinations that would result in duplicates of the same hook. For instance, if a hero is generated with an origin that comes with the Wildheart hook, it is not possible for that hero to have a dote or mote that would also come with the Wildheart hook. In addition, most of the upbringing parts come with so-called forbidden aspects, further making many origin/dote/mote combinations impossible.

This upbringing system results in 1,035,914 unique "regular" hero upbringings and 212,310 unique possible upbringings for children.

Upbringing Stats

Every origin, dote, and mote has a set of "primeStats" and "storyStats".
"primeStats" are a list of stats, such as "Speed, Dodge, Melee Accuracy" or "Potency, Warding". The game assigns a bonus for those stats depending on how many primeStats are set for that specific origin/dote/mote. The more primeStats that origin/dote/mote has, the lower the bonus will be for each individual stat. For instance, origin_36 which has the primeStats Ranged Accuracy, Warding, Speed, Dodge, and Melee Accuracy (5 different primeStats) would give a 0.3 bonus to Warding whereas origin_61 which has the primeStats Ranged Accuracy, Warding, and Charisma (3 different primeStats) would give a 0.5 bonus to Warding.
This means that there is a trade-off, with heroes falling into a spectrum between having a narrow set of very high bonus stats or a very broad selection of comparatively low bonus stats.
"storyStats" are the aforementioned personality stats. Examples are "35 bookish, 35 snark" or "70 poet".

Unique Heroes

Monarchs Under the Mountain

In the Monarchs Under the Mountain campaign, the "Needs Rescuing" farmer has a unique backstory and the following stats:
Hooks: Shame, Gritty, Potential
Potency: +0.2
In addition, that hero has five bonus stat "slots". These "slots" are filled with a randomly selected stat, with a specific bonus per stat. It is possible for the same stat to fill multiple "slots", giving the same stat bonus multiple times, so there is another trade-off between specialization and broadness. Because of this unique generation method, the farmer can obtain stat combinations that are impossibly high for any other hero if enough "slots" are filled with the same stat. Every stat has an equal chance of filling one of the five "slots" with a probability of 1/15. The personality stats are 1-60 for every stat, plus 30 Leader.

Stat bonuses for the Monarchs Under the Mountain farmer
Stat Bonus per "slot" Theoretical maximum (single stat filling all "slots", 1/759375 odds)
Health 0.5 2.5
Speed 0.7 3.5
Recovery Rate 8 40
Retirement Age 4 20
Armor 0.5 2.5
Warding 0.6 3
Dodge 5 25
Block 5 25
Melee Accuracy 3 15
Ranged Accuracy 3 15
Stunt Chance 3 15
Bonus Damage 0.2 1
Potency 0.2 1 (total 1.2 Potency with the guaranteed 0.2 Potency)
Charisma 25 125
Tenacity 25 125
Eluna and the Moth

The non-Legacy starting heroes have fixed stats. It is also possible to recruit Elsee Arcwright, who also comes with fixed stats.

Fixed stats for Eluna and the Moth heroes
Stat Older Sibling Younger Sibling Elsee Arcwright
Hooks Literary, Loyal, Dreamer Flirt, Loyal, Wanderlust Nostalgic, Gritty, Potential
Health 0.6 0.6 1.2
Speed 0.8
Recovery Rate 10 10 7
Retirement Age 5 5
Armor 0.4
Ranged Accuracy
Bonus Damage 0.3
Potency 0.3 0.3
Tenacity 30 30 20
All the Bones of Summer

In the All the Bones of Summer campaign, the non-Legacy starting heroes have fixed stats. In addition, there are two plot-guaranteed recruits, a Hunter and a farmer whose class gets picked by the player.

Fixed stats for All the Bones of Summer heroes
Stat Hunter of the Old Blood The Friend The Toughlipped Kid Fenspear Heir Recruitable Farmer
Hooks Destiny, Wildheart, Nostalgic Nostalgic, Physical, Loyal Gritty, Potential, Slacker Slacker, Literary, Wanderlust Shame, Physical, Thorny
Health 0.5 0.5 0.6 0.5
Speed 0.8 0.7 0.8 0.7
Retirement Age 4 4
Armor 0.5
Dodge 6 5
Block 5
Ranged Accuracy 3
Stunt Chance 3 3 4
Bonus Damage 0.3 0.2 0.2 0.3 0.2
Potency 0.3 0.2 0.2 0.3 0.2
Charisma 30
Tenacity 25
A Walk in the Unlight
Fixed stats for A Walk in the Unlight heroes
Stat An Able Youth Kaylem of Sable Lake Onya of Red Birch Mountain Rayvis the Wroth Daicleed the Disarticulator
Hooks Lucky, Nostalgic, Loyal Dreamer, Literary, Slacker Integrity, Proud, Gritty Distant, Thorny, Potential Weird, Darkheart, Curious
Health 0.8 0.8 1 1
Speed 1 1 1
Armor 0.8 0.8 1
Warding 0.9 0.9 1.2
Bonus Damage 0.4 0.5
Potency 0.4 0.4 0.5

Minmaxing randomly generated heroes

By continuously using the Reroll button until stumbling upon a rare and desirable stat combination, it is possible to minmax.

Some character builds can greatly benefit from certain upbringing stats. Certain upbringing stat combinations make heroes objectively more or less fit to perform certain roles in combat.

For instance, a hero that is generated with 1.1 Speed and 5 Melee Accuracy bonuses from upbringing could be considered optimally suited for a Thornfang Hunter build. A hero that is generated with 10 bonus retirement age is more fit to suit a newly generated Warrior making it through a long campaign without relying on hook quest retirement age bonuses. A hero with a high Dodge and Block bonus is objectively more fit for using Riposte than a hero without.

(Disclaimer) Credits to metoxys for simulating all possible upbringing combinations, analyzing the data, and compiling this upbringing spreadsheet. If you share this around, then please credit metoxys properly.
Upbringing spreadsheet
Upbringing spreadsheet with hooks