As their name would imply, the Fearsome 5 are made up of five Yoshi Kids, each one a different color. The Fearsome 5 are very good friends and are constantly found together, save for when they perform their seemingly common action of playing hide and seek. They are also very mischievous and rebellious, which causes great dismay and stress for their strict babysitter Sushie. The Fearsome 5 are found in the area of Yoshi's Village nearest to Jade Jungle and Mt. Lavalava, sharing their area of residence with Yoshi's Cabana and the local Toad House. If talked to initially, they would all reveal vague notions of mysterious plans they were apparently going to put into action shortly afterward. Indeed, if Mario was to walk to the ledge overlooking Mt. Lavalava's entrance and return, he would find Yoshi's Village in chaos, eventually learning that the Fearsome 5 had somehow managed to sneak into Jade Jungle to play hide and seek. The Village Leader would then persuade Mario and his partners to look for the Yoshi children, a request which Mario complied with. After some searching, the crew eventually found Sushie (who, while looking for the wayward Fearsome 5, had fallen into one of their traps) caught up a tree; after learning that Mario, too, was searching for the Fearsome 5, she decided to temporarily join his party as to double their progress. (Later on in the game, she would make her membership to Mario's party permanent.) The two then continued searching, eventually saving all members of the Fearsome 5 from the various jungle trials they had fallen to. (The only Fearsome 5 members who had not befallen some sort of life-threatening trap were the blue and purple ones (having simply gotten lost in the jungle and fallen asleep in a tree, respectively), who even still realized the error of their ways and regretted past decisions.) In the end, each member of the Fearsome 5 decided that it would've been wiser to have respected Sushie's past orders and ended up travelling back home.
Even though four of the Fearsome 5 were petrified during their experiences in Jade Jungle and initially thankful to Mario for saving them, the Fearsome 5 seemed completely unaffected by their past ventures afterwards. They acknowledged their punishment and seemed to have abandoned all plans of returning to the jungle (despite the fact that they did indeed travel back to it during Paper Mario's ending sequence, as is elaborated on below), but they continued to seem mischievous and even bragged about their adventures, still believing Jade Jungle to be exciting.
The members of the Fearsome 5 appear as playable characters in Mario Rugby League, where they make up a majority of Yoshi's "Yoshi Eggs" team. They make up the team alongside other notable members of the Yoshi species, including Boshi, Yoob, and others.