The Pokémon competitors have a habit of choosing a challenger as a partner while they’re competing. Samus and Pikachu were bonded over the events of subspace emmisary; the two of them have great chemistry, like a classic Pokémon/trainer relationship. Similarly, Lucario and Meta Knight because friends and sparring partners around the same time, and Lucario will usually respond to Meta Knight’s commands. After Charizard split from Pokemon Trainer’s party, it became Lukas’s bodyguard, and they’re relationship has been a huge confidence boost for Lukas. Greninja and Joker seem to have a budding partnership, even if greninja can be standoffish at times. Little Mac was heavily involved in training Incineroar for SSBU while it was just a Litten - which resulted in Incineroar bonding with Little Mac (it’s very affectionate to him, even if he gets singed while getting hugged). The only Pokémon who hasn’t bonded with one particular competitor is… Jigglypuff. However, it can always be seen in the company of the princesses of other games, being pampered and wowing them with its singing voice.
Toon Link and SF64!Wolf to a lesser extent have attempted to bond with it due to them both only joining Smash after the events of Subspace Emissary, but Jigglypuff still isn't particularly attached to them.