With Update 30, in mid June of 2021, ZOS added a new ESO Companion System. These are NPCs that will travel alongside the player and will be a companion that can tank, dps or heal.
Mirri can help any player defeat more difficult enemies and especially newer players have an opportunity to pick up bigger challenges, without having to ask others.
Also, if the group is a player short, Mirri can replace a group member. Meaning if they want to go into a 4-player dungeon and they are only 3 people, someone can summon Mirri and the group will be full.
Mirri won’t be able to do complex mechanics, though, so she might not always be favored over other players.
It is also not possible to summon Mirri while in combat, but perfectly able to unsummon her while in combat.
She will also not be able to be summoned in PvP instances, in single player arenas or in housing instances.
Also, in instances where you can’t navigate to Mirri and talk to her, she will be briefly invisible, such as while swimming.
At this point in time you cannot romance Mirri.
More Guides: Mirri Elendis Leveling Guide