I thought I'd put some suggestions out.
1. Rawk Hawk
How to get: He will be loitering around the Glitz Pit lobby. When you talk to him, he says, "If it isn't The Great Gonzales." Mario mentions his name. Rawk Hawk says, "What!? Thee Mario? No wonder I couldn't best you. I'll admit it was an honor fighting against you. Anyway, I've worked my way back to the top here in Glitzville, but I feel unfulfilled. Empty. I've had my eyes on a specific item for quite some time. What is it? A diamond medallion. The only known one is in and old and spooky place called Creepy Crypt. Boy, what I wouldn't do to get my hands on that. It would absolutely RAAAWK!!" You have now been given a side quest to retrieve this medallion. To get it, you must travel to Gusty Gulch and ask a Boo named Rusty to show you how to get to Creepy Crypt. He will show you a hidden pipe that leads to the outer reaches of Gusty Gulch. Before you go, he gives you the Sun Pendent, for it is the only way to defeat the boss. There you will find a crypt with Dry Bones all around it. Enter and make your way to the inner sanctum. It is here that you must fight the ruler of the Dry Bones, Emperor Marrow. Once beaten, he will drop the diamond medallion. Return to Glitzville and give the medallion to Rawk Hawk. He will say, "I... I don't believe it. The diamond medallion! You not as weak as you look. You are a true hero, Mario. Hmm, ...yes. You've proven such a great feat that I would love nothing more to add some extra muscle to your adventures." --Rawk Hawk has joined your party-- [Rawk Hawk has the ability to lift heavy objects that are impossible for Mario to lift. To do this, stand near the object and press X]. He will finally say, "Let's RAAAWK!!"
Moves: Rawkin' Rush, Dive (2FP), Bear Hug (3FP), Finishing Move (5FP)
2. Fox McCloud
How to get: Go to Twilight Town. Falco, Peppy, Slippy, Wolf, Andrew, Leon, Pigma and Panther will be around the town. Falco will be the first to notice you and say, "You're Mario, right? We don't mean to bother you, but we have an urgent matter on our hands. According to the radar, our team captain, Fox McCloud, crash-landed here. We need you to travel into the woods and find him. Follow the billowing smoke." You must then go through the woods and reach the Creepy Steeple. Fox's Arwing is found wrecked on the ground in front of it. You will hear "AAUUUUUUUUGGH!!!!!" coming from inside the building. Once inside, Fox is found being attacked by Embers. Fox will say, "I can't hold them off. Give me a hand, please!!" After you've fought off the Embers, Fox will say, "That was a close call. Thanks. *?* Hey, Mario. Nice to see you. Anyway, with my Arwing wrecked, It'll be some time before it's repaired. I think I'll tag along with you until then." --Fox McCloud has joined your party-- [Fox can shoot targets, which are required to continue in some parts. Hold X and move the crosshair to the target.]
Moves: Laser, Shield (2FP), Smart Bomb (4FP), Fire Fox (6FP)
3. Sonic (I wanted to find one with the sclera and all, but this was the only one I could find)
How to get: Travel to Petalburg and go to the area with the footbridge that leads to the blue warp pipe. You will find Sonic there. Go talk to him and he'll say, "Mario! Long time no see. Glad to see you're still doing well. Wanna give me a hand here? Actually, It's more of a favor. I need you to travel to Glitzville and get me an Authentic Gourmet Chili Dog. This is no ordinary one. It's of the highest quality and taste and costs 999 coins. Now I know that's a hefty price, but heres the thing. I've left rings around Rogueport for you to collect. Each ring is worth 20 coins. There will be more to compensate you for this favor. See you when you get back." Once in Glitzville, talk to Mr. Hoggle. He will tell you that an important shipment of Gourmet Chili is needed to make the chili dog. It's not a far venture. Talk to Jolene to get access to the storage area. Once there, pick up the chili and head back to the hot dog stand. Authentic Gourmet Chili Dogs are now on the menu there. Mr. Hoggle will tell you that the 999 coin price is a one time thing and that he'll use the money to invest in gathering a supply line of chili. The price will then be set at 60 coins. Return to Sonic and give him the chili dog, which he will proceed to eat. He'll say, "Oh man! That was way past delicious. Thanks for bringing it to me. ...If your going to keep going at a steady pace on your adventures, you'll need a lot of speed. And that's where I come in." --Sonic has joined your party-- [Press X to hang on to Sonic and move the control stick to move at lightning fast speeds. Enemies will get dizzy as he goes by.]
Moves: Spin Dash, Sonic Wind (2FP), Werehog Form (4FP), Super (7FP)
4: Shadow (I couldn't find a paper version)
How to get: You'll find him near the entrance of Forever Forest. If you talk to him, he'll say, "Where is it? Where is the fourth one? *!* Hmpf, I did not expect you to turn up, Mario. I see you've overheard me and ...yes, I do have a dilemma. The fourth Chaos Emerald is lost somewhere in this forest. But no matter which direction I turn, I always find myself back at the start. Think you can prove that your not pathetic? Go in and find the emerald. I'll be waiting." Once you've made your way to Boo Mansion, you'll find the emerald nearby. Retrieve it and take it back to Shadow. He'll say, "I guess I was wrong about you. You're not nearly as pathetic as you look. Hmm... Perhaps you could be of some help to me. I'll accompany you so the rest of the emeralds can be found. And... thanks for finding this one." --Shadow has joined your party-- [Press X to perform Chaos Control and reach places that are otherwise impossible to get to.]
Moves: Punch/Kick, Chaos Control (3FP), Chaos Spear (5FP), Chaos Blast (8FP)
How to get: Krystal will be around the settlement on Keelhaul Key. She'll say, "Oh, hello, Mario. It's nice to see you again. I've been doing fine. I hope Fox is, too. I don't usually ask things like this but can you do me a huge favor? I've heard rumors of an Ancient Krazoa Amulet somewhere on Star Hill (Super Mario RPG). Would you please go get it? I'd be very grateful." Travel to Star Hill and go to the spot where the fourth star piece once stood. Use Flurrie to blow away the hidden part to reveal a pedestal that has the Krazoa Amulet. Unfortunately, this will cause a battle to take place where you must face Ztarz (Black Stars). Once defeated, return to Keelhaul Key and give Krystal the pendant. She'll say, "Thank you so much, Mario. With this amulet, I'll be able to perform more powerful attacks. In fact, I'll need to test them out somewhere. I hope you don't mind if I join you to do this." --Krystal has joined your party-- [Press X to have Krystal send an energy wave from her staff and reveal things that cannot be seen by the human eye.]
Moves: Staff, Aura (2FP), Spirit Energy (4FP), Krazoa Judgement (5FP)
6. Smithy (Super Mario RPG)
How to get: If you revisit the X-Naut Fortress and go into Grodus's Room, you will suddenly be confronted by Smithy, who will say, "Mario! After all this time, We finally come to a second confrontation. 20 years I spent searching for you. 20 YEARS!! I will not let you foil my plans again!" The battle will begin with familiar music playing. Once you have defeated Smithy, he'll say "No... this cannot be. Defeated... again! Curse you! I would have succeeded in rebuilding the Weapons Factory if it wasn't for you." You will mention to him that the moon base could be used to re-create it. "What? This moon base? What are you blabbering about!?" You'll tell him that it would make a perfect domain. "Hmm... Now that I think of it, No one will interfere with me here. You've made an excellent point. However, I will need to regain the proper resources in order to rebuild. I assume you can show me where I might be able to find these thing?" --Smithy has joined your party-- [Press X and Smithy will deal a tremorous blow to the ground. It will be needed in spots where small fissures are.]
Moves: Hammer, Sledge (3FP), Forge (6FP), Change Head (10FP)
I'm always open to more suggestions. If you have any, let me know.