Chansey, the Egg Pokémon, and the evolved form of Happiny. Chansey is a kind Pokémon who shares its egg with those who are sick or injured.
– Dawn’s Pokedex
Chansey is the original healing Pokemon, seen in Pokemon centers acting as nurses and assistants to Nurse Joys. Chanseys are all female, perhaps being the Egg Pokemon, it’s based on mothers.
Chansey is one of the few Pokemon who started off as a single Pokemon, but got a pre-evolution and an evolution later on (in different gens).
Stat-wise, Chansey was the Pokemon with the highest base HP, until it got an evolution in Blissey (who has 5 more base HP). It also has a lot of Sp Defense, making it a great Sp Defense wall. The downside is that Chansey has very little Defense and Attack, 5 base stat each. So any strong physical attack can faint it. However, in Gen V, the item Eviolite was introduced, which is a held item that raises Defense and Sp Defense by 50% if the Pokemon can evolve. So now Chansey with Eviolite can be as good as Blisseys.