When Does Ralts Evolve In Pokemon Platinum?

Ralts evolves at level 20 into kirlia
Kirlia evolves at level 30 into gaudivoir
a male kirlia evolves into gallade through the use of a dawn stone

I evolves in to Kirlia at level 20...
To get a Garvedoir you need to have a female Kirlia n it will evolve at Level 30...
To get a Gallade you need to have a male Kirlia and give it d dawn stone...
D dawn stone is at Survival area go left and you will see a Rocky Wall, use rock climb n battle d dragon tamer and get d dawn stone in an item ball next to him! Gud luck!
To get a Ralts,  :P you need to go to route 208/209 at afternoon time(Day) WEST of HEARTHOME CITY!

Ralts evolves into Kirlia at level 20, and a female kirlia evolves into gaudevor at level 30. A male Kirlia evolves into Gallade if you use a dawn stone on it. (Sorry, do not know where dawn stone is. Try Vielstone city store)