On this page and its subpages, I will attempt to document the entirety of the NiGHTS into Dreams artificial life mechanic, also known as a-life. I'll try to focus on high level information on this page for better readability and discuss technical details in subpages for those who are curious.
Big thanks to Bo Bayles for writing this guide on creating a King Pian and sending me his Ghidra annotations for the rom.
A-life types
In NiD, there are 4 types of a-life. The first is nightopians. These are the starting creatures in the level and have a fixed appearance, with pointed heads, blue shirts, pink wings, orange sleeves, and white shoes. Unlike other types of a-life, they have a great fear of nightmarens and mepians and will fly away when they see them. Their mood will decrease if NiGHTS hits a nightmaren towards them. They also care for their own kind and their mood will drop if NiGHTS paraloops more than 2 of them.
The second type of a-life is mepians. These can only exist with the player's intervention, as they are created when NiGHTS touch dashes a nightmaren and it bounces onto a nightopian. This collision kills both of them, but creates an egg containing a mepian with some traits from each of them. Unlike nightopians, they have no fear of nightmarens or other mepians.
There is one subtype of mepians that have all nightmaren parts and are unofficially known as "badpians." They have all the traits of normal mepians, but they can also eat any a-life they touch, much like full blooded nightmarens can.
The last type of a-life is known as a king or super pian. This pian is created when two mepians mate and random chance causes their offspring to inherit only nightopian body parts. The king pian's behavior is the most unique of all, as it will build a castle in the level it was created in and throws rocks at nearby nightmarens and mepians. They do not reproduce or engage in special behaviors such as singing.
A-life Genetics:
All a-life, whether nightopian or mepian, have certain defining traits located in their DNA. Some of these values determine physical appearance, while others are related to behavior. NiD a-life are haploid (having one copy of a gene) for all traits except the special behavior gene, which is diploid (two copies in the DNA) and the flight pattern gene, which is somewhere in between. This stands in contrast to their successor the chao, which is totally diploid. It simplifies the inheritance of the genes somewhat. More details here.
Non-genetic traits
In addition to traits influenced by DNA, nightopians have a few other traits which are not heritable but change throughout their lives and influence their behavior.
Mood
This is probably the most obvious a-life trait in the game. It influences the level's music and can be viewed directly in the nightopian collection in Christmas NiGHTS. It can range from 0 to 3. Read more here.
Lifespan
A nightopian or mepian inherits a fixed "genetic lifespan" value from one of its parents and will also pass that value on to its children, unchanged. However, there is a second value that keeps track of the actual amount of time the a-life has left from its maximum genetic lifespan. It decreases about once a minute after that nightopian or mepian hatches, meaning with a typical genetic lifespan of 30, a pian can survive for a maximum of 2x30 = 60 minutes as an adult. Most a-life lives much shorter than this, however, because breeding takes 20 minutes off of both parents' lifespan and both mepians and nightopians are at risk of being eaten by predators or paralooped by you, the player.
Desire to mate
This value starts at 0 when a nightopian or mepian hatches and gradually increases over time. When it reaches a threshold (18000 in the game data, which takes about 5 minutes), then the pian will start mating behavior when it finds a suitable partner. What this means differs between nightopians and mepians. Nightopians will only breed with other nightopians (not mepians) who are also ready to mate. Mepians will attempt to mate with other mepians who are ready, but will also pursue nightopians regardless of whether they are ready and will also attempt to breed with nightmarens (which is usually fatal). In any case, once the nightopian or mepian manages to breed with a partner, both parents' desire to mate resets to 0.