Yes, and i think it slightly goes up too.
Other:
They keep their happiness but it can also decrease if you let them faint. Make them have a haircut at the underground path in Pokemon Heartgold or Soulsilver or make them take massages in Veilstone City in Pokemon Diamond, Pearl, or Platinum.
you must do battles and there happiness goes and at some point they evolve.(P.Spokemon that evolve with happiness all evolve at a certain amount or number of happiness.)
To evolve Eevee into Espeon in Pokemon Emerald you need to raise it's happiness to max and level it up during daytime. To raise happiness you can keep the Eevee in your team for a while and fully healed up it should eventually reach max happiness to evolve in the daytime.
The Pokemon that will evolve through full happiness or maximum happiness are Cleffa, Igglybuff, Pichu, Riolu, Togepi, Golbat, Chansey, Azurill and Budew.
It shows how much the Pokemon is happy with you. In some Pokemon you can evolve it if it has high happiness.
You evolve it by raising it to maximum happiness.
You have to evolve it with happiness by batteling other pokemon with him/her
Yes, it will evolve through happiness.
Happiness is gained by taking care of your pokemon giving it items, training it and many more things. Once it's happiness reaches the max happiness have it level up to evolve the pokemon.
To evolve Golbat into Crobat in Pokemon Platinum you need to raise it's happiness to max. To raise happiness simply keep it in your party fully healed for a while. You can also visit a girl in Veilstone City to give it a daily massage to boost it's happiness more. Once it's happiness is at max it should evolve on level up.
Golbat doesn't evolve in pokemon red, blue or yellow. Generation II, and onward, if Golbat's happiness is high enough it will evolve into Crobat.
You'll have to have it have high Happiness in order to evolve it.
You have to evolve it with its happiness level at 220.