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 Oval Stone 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 caught her in a Poké Ball.
In Tanks for the Memories!, Happiny learned how to use Secret Power in order to save Autumn and Ilta from a group of Beedrill. She also used Secret Power later on in the episode to put Team Rocket to sleep so that Brock and Autumn could get away.
Brock was going to use Happiny in her first battle against Marilyn's Shellder in Battling a Cute Drama!. However, right before the battle started, Team Rocket interrupted and tried to steal Marilyn's other Pokémon. Before they could succeed, Happiny was able to send them flying with her powerful Pound attack.
As a Happiny, caring for a sick Pokémon
In Where No Togepi Has Gone Before!, Happiny came out to battle against a mischievous Togepi, as she was immune to her Attract due to both of them being female. Using Pound, she was able to snap the others out of their infatuation and proceeded to keep the Togepi busy until she got tired and left.
In Dealing With a Fierce Double Ditto Drama!, Happiny was used in her first battle by Brock, where she participated in a training round against Narissa's Ditto One and Ditto Two. Both Ditto Transformed into Happiny and used Pound, pushing Happiny back. She retaliated with a Pound of her own, which sent the Transform Pokémon flying into the nearby forest. The battle was called off.
Like most baby Pokémon, Happiny became very attached to her Trainer very quickly. She would become upset whenever she was taken out of Brock's care, and she refused to cooperate with Team Rocket completely. Unlike Bonsly, however, she did not cry or fuss; she actually fought back against Team Rocket's Pokémon after Brock had been hurt.
Happiny had extraordinary strength for a baby Pokémon. Examples of this appeared in Bibarel Gnaws Best!, where she carried many heavy boulders, and in A Maze-ing Race! and Crossing Paths, where she lifted and threw large boulders. She saved Team Rocket, Dawn, and their Pokémon from falling in Journey to the Unown!, and saved Brock and Autumn from falling off a bridge by hurling them high through the air to the other side of it in Tanks for the Memories!. She was also seen walking in Canalave City while lifting Ash's Grotle—who was tired due to its new large size after evolving—with one arm in Jumping Rocket Ship!. She was also able to stop Dawn's Piloswine from rampaging in A Breed Stampede! with her strength. Later, in Leading a Stray!, when Mamoswine tossed Dawn up into the air, Happiny caught her just in time. It is unknown if she has retained her unusual strength following her evolutions, but she was shown to be a competent conventional battler as a Blissey.
The only times where her strength was not enough were in Leading a Stray!, where she failed to pull off a tight manhole cover, in The Psyduck Stops Here!, where she tried to lift a massive boulder but collapsed under the weight, and in Regaining the Home Advantage!, where she attempted to stop a rampaging Metagross but was flung backwards. Any other time, she successfully brought out her special and uncommon power.
In the ending theme Let's Join Hands, she appears as Happiny rather than a Chansey. This also occurs with Steelix appearing as an Onix.
Happiny's enormous strength is ironic, considering Happiny is tied with her evolved form for having the lowest base Attack stats of any Pokémon (at 5).
Blissey is the only Pokémon Brock used in Sinnoh not to have her Ability confirmed.
Blissey is Brock's only Pokémon with three evolutionary stages that were all introduced in different generations. Her Happiny form debuted in Generation IV, her Chansey form debuted in Generation I, and her current Blissey form debuted in Generation II.
Blissey, as a Chansey, is the only Pokémon in the anime to have used the move Soft-Boiled.
Of all the main characters' Pokémon to evolve offscreen, Blissey is the first one to have her evolution revealed in a special episode.
Blissey is the last of Brock's Pokémon to evolve.
Brock’s Blissey is the only Pokémon owned by Ash and any of his friends to have evolved twice from methods that is not solely by level-up.