Hardy doesn't boost or lower any stat. There's almost no reason to keep it in competitive play. Grab a Nature Mint and switch to something that gives your Pokemon an edge. Natures don't exist in Yellow. You'll need a Gen 3+ title to breed for a specific nature.
Hardy is a neutral nature with no stat change. For physical attackers, use Adamant or Jolly instead.
Adamant
+Attack/-Sp. Attack
Maximizes physical damage output
Jolly
+Speed/-Sp. Attack
Outspeed threats while hitting physically
| # | Pokemon | Nature | Tier | Role | Ability | Spe |
|---|---|---|---|---|---|---|
| 1 |  Dragonite DRAGON | Adamant | S | Physical Attacker | Multiscale | 80 |
| 2 |  Gyarados WATER | Adamant | A | Physical Attacker | Intimidate | 81 |
| 3 |  Snorlax NORMAL | Adamant | A | Bulky Support | Immunity | 30 |
| 4 |  Starmie WATERPSYCHIC | Jolly | A | Physical Attacker | Natural Cure | 115 |
| 5 |  Arcanine FIRE | Jolly | A | Physical Attacker | Intimidate | 95 |
| 6 |  Rhydon GROUNDROCK | Adamant | A | Bulky Support | Lightningrod | 40 |
| 7 |  Scyther BUG | Jolly | A | Physical Attacker | Technician | 105 |
| 8 |  Mew PSYCHIC | Jolly | A | Bulky Support | Synchronize | 100 |
| 9 |  Dugtrio GROUND | Jolly | B | Physical Attacker | Arenatrap | 120 |
| 10 |  Shellder WATER | Adamant | B | Physical Attacker | Skilllink | 40 |
| 11 |  Tentacruel WATERPOISON | Jolly | B | Bulky Support | Liquidooze | 100 |
| 12 |  Muk POISON | Adamant | B | Physical Tank | Stickyhold | 50 |
| 13 |  Hitmonlee FIGHTING | Adamant | B | Physical Attacker | Unburden | 87 |
| 14 |  Primeape FIGHTING | Jolly | B | Physical Attacker | Defiant | 95 |
| 15 |  Hitmonchan FIGHTING | Adamant | B | Physical Tank | Ironfist | 76 |
| 16 |  Cloyster WATERICE | Adamant | B | Physical Attacker | Skilllink | 70 |
| 17 |  Aerodactyl ROCK | Jolly | C | Physical Attacker | Rockhead | 130 |
| 18 |  Raichu ELECTRIC | Jolly | C | Physical Attacker | Lightningrod | 110 |
| 19 |  Ekans POISON | Jolly | C | Utility | Intimidate | 55 |
| 20 |  Meowth NORMAL | Adamant | C | Physical Attacker | Technician | 90 |
Hardy is a neutral nature — no stat is increased or decreased.
Competitive Role
No competitive advantage. A stat-boosting nature is almost always preferred.
How to Get
SynchronizeLead with a Hardy Pokemon that has the Synchronize ability. Wild Pokemon will match its nature. (Gen 3+)
BreedingGive the parent with Hardy nature an Everstone. The offspring will inherit it. (100% since Gen 5)
Nature MintUse a Hardy Mint to change stat effects to Hardy. The original nature name stays but stats change. (Gen 8+)
Recommended Alternatives
What does the Hardy nature do in Pokemon?
Hardy is a neutral nature. It doesn't raise or lower any stat, so your Pokemon's stats stay at their base values. In competitive terms, it's the same as having no nature effect at all.
Is Hardy nature good?
How do I get a Hardy nature Pokemon?
Three ways: (1) Breed with an Everstone on a Hardy parent. (2) Lead with a Synchronize Pokemon that has Hardy. (3) Just catch anything. There's a 1/25 chance it already has Hardy. But seriously, you probably want to change it.
When should you use a neutral nature?
Almost never. The only edge case is a Pokemon that uses physical moves, special moves, and needs every defensive stat and Speed. That Pokemon doesn't really exist in competitive. Even mixed attackers prefer Naive, Hasty, or another nature that at least boosts Speed.