Like with every other Pokémon Mystery Dungeon game, you increase your team size by befriending various Pokémon. Typically, this is done throughout the dungeons by defeating the Pokémon. However, there are several Pokémon who will join you on the journey, and are important to the plot as well. They are as follows

Emolga

Description:
This Pokémon, Emolga, is one of the first Pokémon you will obtain. Emolga will join you when you have rescued Dunsparce from two Pawniard who tricked it to go into a dungeon. As it is of the Electric and Flying-types, it can obtain a variety of attacks to help keep coverage up

Emolga
Level 12
Ability:Static
Thundershock
Quick Attack
Tail Whip
Charge
Dunsparce

Description:
When you have rescued Dunsparce, it will join its friend Emolga and become a part of your team. Though it is not the strongest Pokémon, it can help out in some great ways

Dunsparce
Level 6
Ability:Serene Grace
Rage
Defense Curl
Rollout
Virizion

Description:
Virizion will first enter the area early in the game. When every Pokémon falls in love with it, Dunsparce tries to impress it, but when it says it doesn't trust anyone, things go awry. It joins you when it realises that trust can be imbued on other Pokémon

Virizion
Level 20
Ability:Justified
Leer
Double Kick
Magical Leaf
Take Down
Espeon & Umbreon

Description:
Espeon and Umbreon will join you after you have aided them in the collection of Entry Cards to access the MagnaGates

Espeon
Level 20
Ability:Synchronize
Quick Attack
Swift
Confusion
Psybeam
Umbreon
Level 20
Ability:Synchronize
Quick Attack
Pursuit
Confuse Ray
Sand-Attack
Hydreigon

Description:
Originally seen as an adversary, Hydreigon comes to your rescue against Salamence and its Pokémon. Soon after that, it joins you until it valiantly gives its life to protect you from Kyurem. Soon after, however, it is revived and permanently joins you

Hydreigon
Level 64
Ability:Levitate
Dragonbreath
Crunch
Tri Attack
Roar
Keldeo

Description:
Keldeo was looked after by Virizion until it went missing. Found in the Glacier Palace by Espeon and Umbreon, it soon had a hand in protecting everyone from any attacking Pokémon. It joins the team in the post-game

Keldeo
Level 50
Ability:Justified
Take Down
Leer
Aqua Tail
Aqua Jet
Meloetta

Description:
After defeating the game, Meloetta will appear in the Pokémon Paradise and ask for your assistance in creating a Musical Paradise. After this has been created, when you speak to it, Meloetta will join you on your journey. It's Level 15 and it comes with the move Relic Song. When you enter a dungeon, you can select which form you want it to be in

Meloetta
Level 15
Ability:Serene Grace
Relic Song
Round
Quick Attack
Confusion