You have to make it very happy by let him doing easy batttles or give him some medicine. It also help to put him in the first place of your Pokemon party!
These are the best medicine to make your Pokemon happy:
Protein
Zinc
Calcium
Iron
Etc.
You have to do the same thing with a Golbat to get a Crobat ;)
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You could also make it happier by giving it a massage in Veilstone City (its at the first house to the right of the part with the gym) or you could just take it with you all the time
Acttually you should use a shiny stone at about lvl 40-50 and it'll evolve for sure because I did the same to mine.
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The above method of evolving Togetic into a Togekiss is incorrect. Though he is right about Golbat to Crobat by increasing its happiness.
You need a shiny stone to evolve Togetic to Togekiss.
For evolving Togepi, increase its happiness. My togepi evolved at level 14. To increase its happinessa a girl at Veilstone City will massage your Pokemon, increasing its happiness.
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you have to get a shiny stone to evolve togetic to togekiss
Togetic won't evolve in firered only in diamond, pearl and platinum.
Use a shiny stone!
give it the oval stone when it has high friendship
Togetic doesn't evolve at a certain level. You give it a shiny stone and it will evolve into togekiss. It's so super ooper