Cohorts
Cohorts are creatures that, for one reason or another, are with the player characters during an encounter. They count as part of the party when calculating overall encounter level. We must emphasize this, since cohorts can dramatically alter the circumstances of a battle and therefore affect the calculation of an encounter’s difficulty.
Remember that since most skill checks (aside from individual uses in combat as part of a character's actions) can be made by a single character and applied to the entire party, and a party of 3 characters can easily cover all of the bases in terms of skill selection, the addition of extra characters with lower skill modifiers to an already-large party generally has a relatively minuscule effect on encounter balance in non=combat encounters.
The GM, of course, retains discretion whether or not to increase the encounter difficulty if the player characters find a temporary cohort. However, we strongly suggest that temporary cohorts at least access a proportion of the encounter rewards. Simply having a cohort or cohorts should not be a guarantee of steamrolling encounters for cheap loot.
Since cohorts are effectively additional lower-level characters controlled by the players, they can substantially increase the time it takes to resolve combat encounters. GMs who are strongly concerned with speed of play or who are GMing for large groups (particularly groups with 6 or more players) should therefore probably avoid the use of permanent cohorts and may wish to restrict access to temporary cohorts. Additionally, cohorts are usually too much work for beginning players, as beginning players have often not yet learned to play a single character.
On the other hand, cohorts offer a great deal of utility to small groups (such as solo campaigns or groups with only 2 or 3 regular players), and to experienced players who understand and can deal with the mechanical implications of cohorts.
Cohort Rules
Some basic rules apply to all cohorts. The first, and most important, is that cohorts are always at least one level lower than the player characters. We instituted this rule for a number of very good reasons – most importantly, it keeps the story centered around the player characters instead of leaving the player characters overshadowed by an NPC used to advance the plot and create non-interactive "cutscene" play. Additionally, the purpose of creating cohort rules is to duplicate the concept of "leadership," "mentorship," or supply the PCs with dragon mounts and similar awesome things. There's simply no point in having the PCs be in charge of characters that are more powerful than they are, and NPCs that are employing the PCs or otherwise in charge of them should generally be kept off-screen except to provide instructions and quest rewards.
The second rule of cohorts is that a player character may only have a single cohort at any given time.
Finally, cohorts should never be more than 3 levels lower than the players, and 2 levels is generally an unusually wide gap for a permanent cohort. Creatures who are substantially lower-level than the player characters are extremely vulnerable to area effects and the roleplaying value of a permanent cohort cannot be maintained if the cohort is functionally just cannon fodder.