A couple things about the Koopalings:
Larry is really competative. He loves winning above all else and is thrilled that he has others to play against. However, due to a certain breach of conduct in a For Glory match, he has been banned for upwards of 137 years. Even then, no one (and I mean
no one) wants to fight him, so he is an outcast among outcasts (even Waluigi, Ridley, and Chrom are more popular than him!) That being said, he does have a love for all types of sports and spends most of his time in the HRC and Target Blast stadium, watching other characters' records. He even came up to Ganon one time to try to learn from him, but he "is busy with his own agenda." Similarly, he looks up to Little Mac, Ness, and Wii Fit Trainer and wants to learn from them, but none of them will associate themselves with him and have acerbically told him to go away many times. He has actually gotten high up in the HRC score and is at the top of Target Blast (without cheating), but Master Hand throws away his scores so only he knows about it.
Morton Jr. is very cantankerous and grouchy. Overall, most find him unpleasant to be around, though everyone would take him over Larry. Bowser is actually planning on sending him to charm school because he is so grouchy. He is also obese and the largest eater there, even beating Foxy in an eating contest! (Chrom works at Freddy's) This gave him the name "Big Mouth" Koopa.
Wendy is spoiled and materialistic, but otherwise a nice individual...unless your name is Peach, Zelda, Rosalina, Palutena, or Lucina. She constantly engages in "competitions" to see who is the better princess (no one else pays any heed.) Surprisingly, she is good friends with Jigglypuff, as she improved Wendy's singing ability.
Iggy helps with improving and maintaining machinery of other fighters, but this is only to get the blueprints so he may make his own facsimiles for his domination plans. Shulk has seen this and thus will not let Iggy near any of his stuff. Iggy actually managed to fool Master Hand and escape to a world other than his own temporarily (normally smashers are stuck in the world of trophies until they are sent back to their own worlds.) He landed in the world of Twitch Plays Pokemon and met up with Bill. They are now Space Book friends (interdimensional social networking moderated by Francis) and help each other with their plans. Iggy is also
incredibly misogynistic and the biggest pervert there, which doesn't do him any favors in social interaction.
Roy likes fighting and is happy to have worthy opponents for once. Though many call him Bully Koopa, he grew out of his cold-blooded torture phase and prides himself on beating opponents with his muscles alone. He is ticked off that Master Hand forces him to stay in the clown car and on off hours challenges people without it. He is friendly with Little Mac.
Lemmy has stunted development, which makes him physically and mentally like a young child, despite being around 20. He is a goof and hands out with Iggy most of the time. Secretly, all of the Koopalings fear him, as he has the greatest magic potential out of all of them. He only really uses it when he is enraged though. When he does, some say he is more than a match for Dimentio!
once got him mad and had a magical duel with him. It took all of Master Fortress, Giga Bowser, the remaining Mii army, the enemies of Smash Run, the entire fighting cast, and
to mollify Lemmy; now M!Robin cowers as he walks by.
Ludwig keeps to himself, trying to figure out everyone's weaknesses. He learned about how Bowser, Ganon, and Mewtwo in Melee, Tabuu, Mario, Sonic, and Snake in Brawl absorbed Master Core in attempts to take over the multiverse from ROB and is hatching his own plot. He is also the nicest one to Larry and made him "head of the Koopalings" so he can focus on his own plannings. Other than that, he is mercilessly cruel and tortures Bowser Jr whenever he gets the chance, as he found out people cannot die in this world. He is an intellectual rival with Iggy to try and control all.