There are several ways of approaching happiness, some universal, while others depend on the game.
- Give it a Soothe Bell. (+1 to each of the following events)
- Capture the Pokemon with a Luxury Ball. (+1 to each of the following events)
- Take it along with you. (+1)
- Never leave it in your box or at the daycare.
- Never trade the Pokemon! This resets its happiness!
- Keep it alive during battle.
- Leveling it up. (+2)
- Giving it massages/haircuts. (+2-5)
- Giving it Lemonade, Fresh Water, Soda Pop (+2)
- Feed it vitamins
- Feed it EV-reducing berries.
- Feeding it Poffins/PokeBlocks that it enjoys (Modest Pokemon enjoy Blue PokeBlocks and Dry Poffins)
The Soothe Bell is an item that usually appears in each game somewhere or somehow. One of those items that you usually don't look at twice, it helps Pokemon become happier, as long as the above parameters are met.
Luxury Balls are rare, but greatly increase a Pokemon's happiness. You may only find one or two in the game, so be decisive!
Bringing your Pokemon along with you is a sure-way of getting it to like you. Never leave it in your box or at the daycare, or it will decrease it's happiness. And for the love of god, never NEVER NEVER EVER trade your Pokemon with a friend. This will reset it's happiness and you'll have to start all over.
Giving it potions and healing it before it faints helps to keep your Pokemon happy.
Leveling up and allowing the Pokemon to grow will also increase its happiness.
Masseuses can be found around each region. There's one in Pallet Town, Goldernrod City, Veilstone City, and the Ribbon Syndicate in each respective game. You can only receive once a day.
Some items like EnergyPowder, Heal Powder, EnergyRoot and Revival Herb will reduce happiness. These items, though categorized as bitter, will not appeal to Pokemon that enjoy Bitter foods. It will still lower their happiness. Stay away and use them only on emergency situations.
Poffins and PokeBlocks will raise a Pokemon's happiness, depending on if they like them. Research the Pokemon's taste to figure out what it likes and dislikes.
Some berries, like Pomeg, Kelpsy, Qualot, Hondew, Grepa, and Tamato will lower EVs in exchange for happiness. Not recommended, but worth it in the end.
There are many methods of discovering your Pokemon's happiness. In FireRed/LeafGreen, the Pokemon appears close to the screen when it likes you, and at max happiness, it's cry can be heard when an item is used.
In Diamond, Pearl and Platinum, the Friendship Checker allows you to select a party Pokemon and see its friendship. At max friendship, it will jump up and down and emit its cry.
You can also visit Pokemon Readers, usually found in each region. They will provide you with a blurb about your Pokemon's happiness. There is one in Verdanturf City (RSE), one in Pallet Town (FRLG), one in Goldenrod City (HGSS), Iccuris City's Pokemon Fan Club (BW), and Hearthome City's Pokemon Fan Club (DPPt). There are also others who will give you items depending on happiness. In Pacifidlog Town, a man will give Return and Frustration once a week to trainers who present a Pokemon with max or low happiness respectively. This will also occur in the Goldenrod Department Store in the TM Corner. Dr. Footprint will reward a Pokemon with max happiness a ribbon. He can be found on Route 213.
Your Pokemon has Max Happiness if the reader tells you the following:
BW: It is super friendly to you! I'm a bit jealous!
DPHGSS: It looks really happy! It must love you a lot
Plat: It simply adores you! Why, I feel like I'm intruding!
FRLG: It couldn't possibly love you any more than it does now. Your Pokemon is happy beyond words.
RSE: It adores you. It can't possibly love you any more. I even feel happy seeing it.
At High Happiness, they will say:
BW: It is quite friendly to you! You must be a kind person!
DPHGSS: I get the feeling that it really trusts you
Plat: It's very friendly toward you. I can tell you treat it kindly.
FRLG: It looks very happy.
RSE: It seems to be very happy. It obviously likes you a whole lot.
At Moderate friendship, they will say:
BW: It is friendly to you. It must be happy with you.
DPHGSS: It's friendly toward you. It looks sort of happy
Plat: It's quite friendly to you. It must be happy being with you.
FRLG: It's quite friendly with you. Keep being good to it!
RSE: It likes you quite a lot. It seems to want to be babied a little.
At Acquaintanceship, they will say:
BW: It is a little friendly to you... That's what I'm getting
DPHGSS: It's quite cute
Plat: It's warming up to you. That's my impression.
FRLG: It's warming up to you. Trust must be growing between you.
RSE: It's getting used to you. It seems to believe in you.
At Neutral Friendship, they will say:
BW: The relationship is neither good nor bad... It looks neutral.
DPHGSS: You should treat it better. It's not used to you.
Plat: It's feeling neutral toward you. It's up to you to change that.
FRLG: It's not quite familiar with you yet.
RSE: It's not very used to you yet. It neither loves nor hates you.
Finally, at low to nonexistent friendship, they will say:
BW: Hmmm... It may not like you very much.
By any chance, you... Are you a very strict person? I feel that it really doesn't like you...
DPHGSS: It doesn't seem to like you at all. It looks mean
Plat: Hmm... It seems to dislike you a little.
FRLG: I don't like the way it glares at you. Could you try being a little nicer to it?
RSE: It's very wary. It has scary viciousness in its eyes. It doesn't like you much at all.
It's funny...the last options make you look like an abusive trainer. Many Pokemon, including legendaries, start out with low base happiness. With low Happiness, the move Frustration is at its strongest. When a Pokemon has zero happiness, Frustration's base power is at 102. This goes for Return, when happiness is at its maximum.
The following Pokemon require max happiness to evolve:
* ** ** ***
* Eevee can evolve into two different Pokemon through happiness: Umbreon and Espeon. Umbreon requires happiness at afternoon/night (12 PM to 12 AM) while Espeon requires it during the morning/day (12 AM - 12 PM)
** Must be during day.
*** Must be during night.
Happiness Guide at Bulbapedia