Both the quality of consumables a monster produces and the amount of money the clients are willing to pay for the monsters increases based on their stats.
The best way to increase a monster's stats is by mating them.
"Monster breeding works nothing like it does with regular animals. Monsters aren't actually born in this world, they're drawn forth from the Chaos that borders existence. When the sperm of one monster penetrates the ovum of another, a portal opens up in that monster's womb. The [fertilized] ovum gestates and is born in the Chaos, where time flows separately from [the] world. The offspring actually spends what it experiences as dozens to hundreds of years living in the chaos until it is pulled back through the portal to [the] world". - Cordelia
There are five modifiers that are effected when breeding monsters. These modifiers effect the benefits the paired monsters receive by mating, as well and the chance of producing offspring.
A monster's level is increased by breeding. Breeding a monster gives it experience points (XP). Once an experience threshold is reached, the monster's level increases.
Typically speaking, the higher a monster's level, the more XP is grants its lower leveled partners. However, traits can modify this behavior.
A monster's happiness is determined by breeding. Breeding a monster increases it's happiness, while neglect can lead to a loss of happiness.
If a monster's accumulated hearts spill over the maximum, the monster's happiness level decreases. Harvesting the monster before the hearts exceed the max prevents this from happening.
Monsters have preferences that give them more happiness when bred with certain monsters, and can develop new preferences over time.
Breeding a monster with its preferred partner type will raise that monster's happiness higher.
Each stat has it's own experience level. The experience level indicated how they will affect their partner's stats when they have sex.
- Strength: (STR) increases the XP the other monster is gaining while mating.
- Dexterity: (DEX) reduces the amount of energy that it takes to breed or harvest a monster
- Stamina: (STA) increases the rate at which a monster generates hearts.
- Charisma: (CHA) increases the amount of experience the monster gets from breeding.
- Will: (WIL) increases the number of hearts a monster can store.
- Fertility: (FER) increases the chance that breeding will result in a new monster.
Harvesting a monster will produce a consumable for each stored heart. Consumables can also be bought or sold in town. Feeding a consumable to a monster will increase the monster's stats based on the consumables quality.
Additionally, consumables have special effects depending on what type of monster the consumable came from. These effects can be incredibly strong when used intelligently.
Every time the player breeds a male and female monster together, there's a chance the female will give birth. Monster reproductive systems are absurdly quick: she'll give birth right there on the spot.
Some monsters have special traits that influence their stats or appearance. These, along with breeding and stat ratios, can be passed from parents to their offspring.
At any one time, a breeder will have clients available to them. The clients can be bronze, silver, gold, or platinum level. Silver clients pay better than bronze, but are pickier about the quality of their monsters, and gold clients both pay and demand more than either of them.
Platinum clients pay best of all, but are looking for monsters for distinct purposes - they will wait longer for the right monster.
Whenever the player completes a gold or platinum level task, the guild will give him/her permission to raise monsters with new special traits, and they'll become available in the shop.