Brock won Happiny's egg in a Pokémon cosplay contest in All Dressed Up with Somewhere to Go! by entering his Croagunk dressed up as a Politoed. He was overjoyed to take care of the egg, as he showed in the next episode when he rubbed his cheek against it affectionately.
In One Big Happiny Family!, while traveling on Cycling Road, the egg began to glow. After taking it to a Pokémon Center and receiving assistance from Nurse Joy, the egg hatched into Happiny. Soon after birth, she began crying for a stone to hold as a fake egg. Brock found a white stone and polished it into an egg-like shape for her. Team Rocket snatched Happiny away before he could give the stone to her. Eventually Happiny was returned to Brock and received her "egg." Immediately after, she offered it back to Brock as a sign of friendship. He gave it back to her and then captured her in a Poké Ball.
Like most baby Pokémon, she became very attached to her Trainer very quickly. She was upset when taken out of Brock's care, and refused to cooperate with Team Rocket completely. Unlike Bonsly, however, she did not cry and fuss; she actually fought back against Team Rocket's Pokémon after Brock had been hurt. Happiny has a cheerful personality, and is playful and energetic. Rather than walking, she is usually held in Brock's arms, much like Misty did with her her Togepi. She also is sometimes seen sleeping or playing with Sudowoodo. Sometimes she is seen riding on Pikachu's back, as seen in A Maze-ing Race! and A Lean Mean Team Rocket Machine!.
Happiny has extraordinary strength for a baby Pokémon. She'd knocked Meowth, Mime Jr., and Wobbuffet out of her way and flung Jessie's Seviper by the tail so she could get back to her trainer. She also launched Brock several meters into the air. She is very playful and has a lot of energy. Happiny carried many cement blocks in Bibarel Gnaws Best! and threw a big boulder in A Maze-ing Race!, Crossing Paths, and The Psyduck Stops Here! . In Losing Its Lustrous, she happily dumped everything out of Brock's backpack. In Steamboat Willies, she saved Piplup, Pikachu, and Pachirisu from falling off a steamboat by happily tossing them back on board, help save Team Rocket, Dawn, and their Pokémon from falling in Journey to the Unown!, and saved Brock and Autumn from falling off a bridge by chucking them high in the air to the other side of the bridge in Tanks for the Memories!. She ran around with a Quagsire in her hand in The Rise of Darkrai. Similar to this scenario, she was also seen walking in Canalave City whilst lifting Ash's Grotle with one arm in Jumping Rocket Ship who was comically tired due to its new form. She was briefly shown holding a large globe at the end of Giratina and the Sky Warrior. She was also able to stop Dawn's Piloswine from rampaging in A Breed Stampede! with help of her strength. In DP109, she tried to pull off a tightly manhole cover (but failed), and when Mamoswine tossed Dawn up into the air, Happiny caught her in time. However though, in DP087 , she tried lifting a massive boulder, but collapsed under the weight and was knocked out.