All characters including enemies and player characters are, broadly speaking, monsters (which is to say, they appear in the monster editor). Characters available on the character selection screen are 240-247. Guest characters start at 225. Apollo (254) and Arsenal (255) have special hard-coded behaviors associated with them. Some item scripts also have special behavior based on monster id, generally in the form of providing some form of immunity to monsters with high ids (which are generally bosses), but these are implemented inconsistantly. (Some more details are available in the technical document.)
Larger monster inventories take more space to encode. Saga2edit shows a report about this space usage at the bottom of the Monster Editor window.
Monster names are limited to 8 characters (technically, 8 bytes, but saga2edit won't use digraphs to encode monster names). Escape sequences (as used in scripts) are enclosed in parenthese and can be used to encode things like icons. Saga2edit won't accept script instructions in names, (but they could be encoded manually using hex escapes).
There are four types: human (0), mutant (1), monster (2), and robot (3).
These are the monster's statistics. (For player characters, these are the initial values which change over the course of the game.) These values are inclusive of any bonuses from equipment. (For example, Human M starts with Bronze Armor and 3 defense. Unequipping the armor will leave him with 0 defense.)
These settings control the appearance of enemies in battle. There are 9 regions each with up to 16 appearances. (There is space to encode 16 different regions, but only 9 are used.) Each region contains graphics of a particular size (although common sizes can have multiple regions associated with them).
This controls the appearance of player characters in the map screen and in menus.
This controls the amount of GP an enemy leaves behind when defeated in battle. The game uses a look-up table for these, so only certain values are available.
The family setting is mainly used as part of the monster transformation logic. There are also certain attacks that have different behaviors for certain families.
This is the species index within the family. This is used as part of the monster transformation logic. (Sometimes, these are referred to as A (0), B (1), and C (2).)
Enemies with this flag will drop meat, and enemies without it will drop items from their inventory. (Abilities can't be dropped.)
These flags control the chance that an enemy will drop meat or an item (as determined by the meat flag). If neither flag is set, the enemy won't drop anything. If the rare drop flag is set, the chance is about 20% (but goes up if there are more enemies in the stack). If the common drop flag is set, the chance is about 80% (although the vanilla game never uses this flag alone). If both flags are set, the drop is guaranteed (although enemies that drop items may still fail if they have non-droppable items).
Enemies' level (measured against the player characters' power) affects the rate of advancement for human and mutant characters. This setting is also involved in the monster transformation logic. Many sources refer to this as DS level.
This controls the enemy's chance of using each item in its inventory. The value is an index into a lookup table, but in practice, a value here indicates that items between 0 and this value (inclusive) can be selected in battle.