Companion

Companions or followers are the allies of player characters who accompany and aid them during their adventures.

Their equivalent in Fallout Tactics are called recruits.

Controls
Companions cannot be controlled directly. Instead, one can change some settings for their AI. In Fallout, the controls are rudimentary at best while Fallout 2 is a substantial improvement.

Level up
Companions don't level up by gaining experience as the protagonist does.

In Fallout, companions do not level up at all.

In Fallout 2, each companion has a fixed number (typically, 4-6) of predefined levels called "stages" in the form of multiple alternative models for them with different statistics. Each time the player character gains a level, every current companion has a chance to go up a stage i.e. change to a more powerful model. A companion starts with a base model, then advances to stage 1, 2 etc.

Using fixed models also means that a companion cannot permanently gain stats in any other way, e.g. by reading skill books.

Due to a design bug, companions who are supposed to have six stages cannot ever reach Stage 6, as their progression is divided not into six, but seven stages. For example, Goris is supposed to have six stages (protos 319 through 324), but the character the player interacts with and recruits is none of these protos, but a separate, "base" Goris (proto 153).

This would not normally be an issue, but each record in the file is capped at 200 bytes of length, which only accommodates five records. Anything exceeding that length is truncated and ignored by the engine. As such, each six-stage companion can only level up to Stage 5, as the data generated from party.txt at startup will never include the sixth stage, due to a hardcoded limit.

This was not a well-known bug, even to developers: In Fallout Bible 4 Chris Avellone only showed the stats for stage prototypes and missed the base prototype for Goris.

Usable weapons
The kinds of weapons a companion can use is not determined by the weapon's type but rather by the in-game model the weapon utilizes when used. A companion can use weapons that their character model has animation sprites for. The types of weapon models are:

It's due to this mechanic that a companion's model doesn't change from wearing different types of armor; changing the model would change their qualifications as well. The added benefit here is making it easier to distinguish the companion from regular NPCs wearing the same type of armor.

Active skills
In Fallout 2, if a companion has a higher active tag skill than the Chosen One, whenever they use that skill on something, the companion lends them a hand by running in and using the corresponding skill on the object themselves. This way, a skill check will be done against their skill level, which allows the Chosen One to do things they would otherwise be not qualified to.

This doesn't, however, extend to other interactions like skill checks in normal dialogue and during regular object use (e.g. it's not possible to use a strong companion to open jammed doors in Vault 8), or when using items that act the same as an active skill (e.g. a doctor's bag). This severely limits companions' usefulness as specialists, as they can't really save the protagonist from having to train all the needed skills and stats themselves.