Party member

Party members, also known as partners or allies, are characters that travel with and battle alongside the protagonist in RPGs. The protagonist and his partners as a group are generally referred to as a party, as the event of a character joining is referred to as said character "joining your party."

Super Mario RPG: Legend of the Seven Stars
In Super Mario RPG: Legend of the Seven Stars, Mario's party eventually gains five members (including Mario himself), but only three of them can be active at any given time, and Mario must always occupy the first slot. Inactive party members still gain experience and level up after battles.

Paper Mario
In Paper Mario for the Nintendo 64, partners are either referred to as Party Members or Mario's Friends. In this game, Mario can only have one Party Member active at a time. Switching Party Members mid-battle uses up Mario's turn unless he is wearing the Quick Change badge. Party Members do not have HP; most enemies' attacks will only target Mario, and the few that can hit Mario's Party Members will simply incapacitate them for a certain number of turns depending on how much damage they took. If the attack is successfully guarded against, the Party Member will take no damage. Party Members can be upgraded with Super Blocks, which increases their attack power and makes them learn new moves. Each Party Member can be upgraded twice, first to Super Rank, then to Ultra Rank. Mario must have the Ultra Stone to upgrade Party Members to Ultra Rank.

Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, Mario's party members are referred to as partners or allies. The party mechanics work similarly to the first Paper Mario, but with several changes. Partners now have HP and take damage the same way Mario does. Partners are now upgraded by giving Merlon three Shine Sprites.
 * Goombella - An archeology student at the University of Goom, Goombella is searching for the same Legendary Treasure as Mario and decides that it is in their best interest to work together. Goombella has 10 HP initially, 20 at Super Rank, and 30 at Ultra Rank.
 * Headbonk - Headbonks any ground or flying target (0 FP)
 * Tattle - Will analyze any enemy (discovering their HP, Attack and Defense), and will open text about the enemy (0 FP)
 * Multibonk - Headbonks any target on the ground or flying multiple time (3 FP, Super Rank)
 * Rally Wink - Gives Mario another chance to act (4 FP, Ultra Rank)
 * Koops - Koops's father hadn't been seen since he'd gone off to challenge the ferocious dragon Hooktail. After hearing that Mario and Goombella planned to launch a fresh assault, he decided that he wanted to join them to prove to everyone (particularly his girlfriend Koopie Koo) that he could be tough like his dad.  Koops has a Defense of 1, but can be knocked on his back like enemy Koopas.  He has 10 HP initially, 15 at Super Rank, and 25 at Ultra Rank.
 * Shell Toss - Shoots Koops's Shell at the closest ground enemy (0 FP)
 * Power Shell - Strikes all ground enemies with Koops's Shell (3 FP)
 * Shell Shield - Provides a temporary shell to defend Mario (4 FP, Super Rank)
 * Shell Slam - Attacks all ground enemies and penetrate their defense (6 FP, Ultra Rank)
 * Flurrie - Madame Flurrie was a retired actress, just dreaming of coming out of retirement when Mario and friends asked for her help with a small task. She found their stories of adventure alluring, and decided that she would come with them to help as much as she could.  Flurrie has 15 HP initially, 25 at Super Rank, and 35 at Ultra Rank.
 * Body Slam - A heavy attack on any enemy (0 FP)
 * Gale Force - Attempts to blow all enemies out of battle (4 FP)
 * Lip Lock - Flurrie kisses any enemy, dealing damage that penetrates Defense and healing herself the same amount (3 FP, Super Rank)
 * Dodgy Fog - Makes Mario Dodgy for a few turns so that he may evade enemy attacks (4 FP, Ultra Rank)
 * Yoshi - This baby Yoshi hatches from an egg that Mario found outside the Glitz Pit. He could be any one of a variety of colors and the player can name him.  He has 10 HP initially, 20 at Super Rank, and 30 at Ultra Rank.
 * Ground Pound - Hits any ground or flying enemy repeatedly for 1 damage each hit (0 FP)
 * Gulp - Swallows an enemy then spits it at another, damaging both; this move was the only way to defeat the Armored Harriers (4 FP)
 * Mini-Egg - Deals relatively low damage, but hits multiple foes and can also shrink enemies to reduce their power (3 FP, Super Rank)
 * Stampede - Summons a Yoshi herd to attack all enemies; hits multiple times for 1 damage each (6 FP, Ultra Rank)
 * Vivian - Once a member of the villainous Shadow Sirens, Vivian decides that Mario is much more deserving of her help than her sister Beldam is. She has 15 HP initially, 20 at Super Rank, and 30 at Ultra Rank.
 * Shade Fist - Punches any enemy and also burns the target (0 FP)
 * Veil - Vivian and Mario disappear into the shadows and can't be hit by attacks (1 FP)
 * Fiery Jinx - Blasts all enemies with fire damage that penetrates Defense and leaves them burned (6 FP, Super Rank)
 * Infatuate - Confuses all enemies (4 FP, Ultra Rank)
 * Admiral Bobbery - Admiral Bobbery had refused to go out to sea after a tragedy in his past. Mario must learn his secret to convince him to serve as navigator on a journey to Keelhaul Key, and he joins Mario's party shortly after the ship's crash.  He has 20 HP initially, 30 at Super Rank, and 40 at Ultra Rank.
 * Bomb - Damages the closest ground enemy (0 FP)
 * Bomb Squad - Tosses three time bombs that explode later (3 FP)
 * Hold Fast - Damages enemies that attack Bobbery directly for a few turns (4 FP, Super Rank)
 * Bob-ombast - Deals serious damage to all enemies (9 FP, Ultra Rank)
 * Ms. Mowz (optional) - A dashing badge thief, Ms. Mowz and Mario cross paths a number of times before she asks him to find a badge for her. Unknown to him, it was a test of his abilities--and once he passes and proves himself to her, she'll join the party and lend her treasure-sniffing abilities to the heroes.  She has 15 HP initially, 20 at Super Rank, and 25 at Ultra Rank.
 * Love Slap - Damages the closest ground enemy, ignoring Defense (0 FP)
 * Kiss Thief - Steals the closest ground enemy's item or badge (2 FP)
 * Tease - Makes enemies Dizzy (3 FP, Super Rank)
 * Smooch - Heals Mario 10 HP (10 FP, Ultra Rank)

Characters

 * Mario - The first of the four Heroes of Light, Mario can Flip between dimensions.
 * Princess Peach - Joins Mario when she discovers she is a hero of Light. She can duck under her parasol, blocking most attacks, and she can also use it to float through the air.
 * Bowser - Joins Mario when he is convinced by Peach. He has twice the Attack of Mario, Peach & Luigi and can breathe fire, but he is larger and slower than the others and can't jump as high. He is also unable to climb ladders.
 * Luigi - Joins Mario when found in The Underwhere; can jump the highest of all the heroes and can also use his super jump technique to jump even higher and to deal double damage.

Pixls

 * Tippi - Allows the player to Tattle enemies and reveal invisible objects.
 * Thoreau - Allows the player to pick up and throw enemies and objects, as well as activate switches.
 * Boomer - Acts as an explosive.
 * Slim - Allows the player to turn sideways to move through narrow spaces and evade attacks.
 * Thudley - Allows the player to perform a Ground Pound.
 * Carrie - Can carry the player on a floating platform, allowing them to move faster, bypass spiky floors, and jump on spiked enemies.
 * Fleep - Can flip sections of the background, as well as enemies to make them briefly dizzy.
 * Cudge - Acts as a Hammer.
 * Dottie - Can shrink the player, allowing them to avoid detection by enemies and fit through small doors, pipes, and passages.
 * Barry (Optional - Unlocked by completing Chapter 3 and returning to the start of it and speaking to him) - Projects a barrier around the player.
 * Dashell (Optional - Unlocked by completing Flipside Pit of 100 Trials) - Increases the characters' speeds.
 * Piccolo (Optional - See Piccolo for unlocking instructions; a long list of errands) - Can use music to heal status ailments.
 * Tiptron (Optional - Sold by Francis after Chapter 8's completion for 999 coins) - Replaces Tippi after beating the game.

Paper Mario: The Origami King
In Paper Mario: The Origami King all party members are temporary, only accompanying Mario for specific periods of time or in specific streamer districts. Party members only have one attack in battle and are AI controlled.

Battle allies

 * Bob-omb
 * Spike
 * Bone Goomba
 * Professor Toad
 * Kamek
 * Bowser Jr.
 * Bowser

Mario & Luigi series

 * Mario
 * Luigi
 * Baby Mario
 * Baby Luigi
 * Bowser
 * Popple (during one fight with Wiggler)
 * Prince Dreambert
 * Paper Mario
 * Nabbit (during some fights in the Bad Nabbit mission)

Trivia

 * Luigi also gets partners in Paper Mario: The Thousand-Year Door: Blooey, Jerry, Torque, Hayzee, and Screamy. However, the player cannot play as them, for Luigi is not a playable character in the game.