Companion Characters (5e Other)
From D&D Wiki
A companion character is an NPC that a DM can have accompany the player's characters in order to fill a missing role. They are useful if your regular group is temporarily missing a player, or you are running an adventure designed for a larger group.
While the DM could create a NPC using the classes presented in the PHB, companion characters are simpler for two reasons. Firstly, the gaming group might not want a party NPC to equal or overshadow the other PCs in power. Secondly, the NPC should not overburden the controlling DM or player with additional choices or circumstantial benefits to keep track of; this includes not giving a companion any features that use their Reaction or points that need to be kept track of.
Companion characters are considered to be a PC for the purpose of designing encounters and awarding XP. They may take a share of treasure rewards, but do not take magic items unless offered by the PCs.
Use the tables in the Creating Nonplayer Characters chapter of the DMG to round out the character.
In addition to the equipment granted by their companion class, companion characters have a generic "backpack" that contains whatever food, ammunition, torches, utensils and other mundane items they require for their role. It is assumed they replenish consumables by themselves, unless the narrative suggests otherwise (lost in a desert...)
Ability Scores and Hit Points
Ability scores and hit points are pre-calculated as shown in the class tables, so that a companion can be immediately included in a game session.